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Isothermal scale model study of the gas flow field in a hog fuel boiler furnace Perchanok, Mathias Samson 1988

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ISOTHERMAL SCALE MODEL STUDY OF THE GAS FLOW FIELD IN A HOG FUEL BOILER FURNACE By MATHIAS SAMSON PERCHANOK B.ASc, The U n i v e r s i t y of Waterloo, 1985 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE / i n THE FACULTY OF GRADUATE STUDIES THE DEPARTMENT OF MECHANICAL ENGINEERING We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May 1988 ® Mathias Samson Perchanok, 1988 In Presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference or study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the Head of my Department or by her represntatives. I t i s understood that p u b l i c a t i o n , i n part or i n whole, or the copying of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Mathias Samson Perchanok Department of Mechanical Engineering The U n i v e r s i t y of B r i t i s h Columbia Vancouver, B r i t i s h Columbia Canada, V6T 1W5 Date ABSTRACT Good combustion i s required to maximize steam generation and avoid emissions i n the wood waste f i r e d b o i l e r s found frequently i n the pulp and paper industry. The combustion process i s a s s i s t e d i f v e l o c i t i e s above the grate are minimized, i f gases i n the combustion zone are mixed intimately, and i f gaseous combustion i s concentrated above the grate. A 1/15 scale r e p l i c a of a power b o i l e r was constructed for isothermal flow modelling of the flow f i e l d above the grate, the o v e r f i r e a i r j e t s and the furnace up to the generating banks. The flow was made v i s i b l e with smoke, and v e l o c i t i e s were measured with a pulsed wire anemometer. I t was found that a very non uniform v e l o c i t y p r o f i l e occured above the f u e l grate because the under grate plenum d i d not adequately d i f f u s e the under grate a i r flow. Also, non perforated areas of the f u e l grate near the furnace walls caused r e c i r c u l a t i o n zones to occur above the grate near the f r o n t w a l l . The r e c i r c u l a t i o n zones of the o v e r f i r e a i r j e t s caused high v e l o c i t i e s to occur above the grate as w e l l . A large space between the f r o n t w a l l and the f r o n t o v e r f i r e a i r nozzles on the side walls, caused v e r t i c a l s t r a t i f i c a t i o n of the flow, i n which gases r i s i n g from the grate, near the front w a l l d i d not mix with the o v e r f i r e a i r . At part load, reduced v e l o c i t y i n the o v e r f i r e a i r j e t s caused v e r t i c a l s t r a t i f i c a t i o n of the flow as w e l l , i n which mixture of gases r i s i n g from the grate and o v e r f i r e a i r occured well above the f u e l grate. At high i i i nozzle v e l o c i t i e s , gases were well mixed, throughout the cross section, large r e c i r c u l a t i o n s occured and mixing was concentrated near the f u e l grate. Established models f o r j e t s f o r free turbulent j e t s do not accurately represent opposing banks of j e t s . Centerline v e l o c i t y i s overpredicted by a f a c t o r of two or more, and d e f l e c t i o n of the j e t are g r e a t l y underpredicted by the models. Throw and penetration, c a l c u l a t e d with the models do not give reasonable p r e d i c t i o n s , i n d i c a t i n g the need f o r more so p h i s t i c a t e d models. At high o v e r f i r e a i r flow rates, o s c i l l a t i o n of the o v e r f i r e a i r j e t s was observed, with a period i n the order of one second. i v TABLE OF CONTENTS Section Page 1. INTRODUCTION 1 2. BASIC ELEMENTS OF A HOG FUEL BOILER 4 3. RESULTS OF SURVEY OF THEORY OF COMBUSTION IN . . . 7 HOG FUEL BOILERS 4. MODELING OF FURNACE FLOW FIELDS 12 4.1 Review of Methods 12 4.2 I d e n t i f i c a t i o n of Non Dimensional Parameters . . 16 Relevant f o r Isothermal Modeling 4.3 Derivation of Scaling Laws for the 20 Isothermal Model 5. THE APPARATUS 25 5.1 The Model 26 5.2 A i r Flow Systems 27 5.3 Pulsed Wire Anemometer Equipment 29 5.4 Smoke Flow V i s u a l i z a t i o n 34 6. EXPERIMENTAL PROCEDURES 35 6.1 Model A i r f l o w Settings 35 6.2 P.W.A. Procedures 36 6.3 Smoke Flow V i s u a l i z a t i o n 37 7. RESULTS 39 7.1 Model A i r Flow Rates 39 7.2 Plots of the P.W.A. Results 41 7.3 Smoke Flow V i s u a l i z a t i o n Results 44 8. DISCUSSION OF RESULTS 47 8.1 Gas Flow Above the Grate 47 8.2 Mixing 58 V 8.3 Aerodynamics 83 8.4 Buoyancy E f f e c t s 93 9. CONCLUSIONS AND RECOMMENDATIONS FOR HOG FUEL . . . . 96 BOILER DESIGN AND MODIFICATION BIBLIOGRAPHY 102 APPENDIX A INSTRUMENTATION . . . 105 A . l L i s t of Instruments 105 A.2 C a l i b r a t i o n of Instruments 106 A.3 P r i n c i p a l Sources of Error 108 A.4 Obtaining Accurate and Repeatable 110 Measurements with the P.W.A. A. 5 INTEGRATION OF P.W.A. RESULTS TO 114 OBTAIN MODEL FLOW RATES APPENDIX B THE WOODFIBRE #4 POWER BOILER 116 B. l General Description of the B o i l e r 116 B.2 Combustion C a l c u l a t i o n 118 B. 3 C a l c u l a t i o n of Gas Flow Rates 119 and Densities i n the Furnace APPENDIX C SURVEY OF THEORY OF COMBUSTION IN HOG 126 FUEL BOILERS C. l Combustion of Wet Wood and Bark 127 C.2 Combustion of Fuel on the Grate 128 C.3 Gaseous Combustion . . 135 C.4 Combustion of Entrained Fuel P a r t i c l e s . . . 141 C.5 Aerodynamics 151 C.5.1 Overfire A i r J e t Models 152 C.5.2 R e c i r c u l a t i o n Zones and 162 Channelling i n the Flow C.5.3 Buoyancy E f f e c t s 164 v i C.5.4 Conclusions about Aerodynamics . . . . 166 i n Furnace Flow F i e l d s APPENDIX D PROPERTIES OF HOG FUEL 168 APPENDIX E EXPERIMENTAL RESULTS 171 v i v i i LIST OF TABLES Table Page 7.1.1 Flow Rates and Scaling Factors i n the 41 Prototype Furnace and the Model 8.2.1 O v e r f i r e A i r Jets and Mixing 81 8.3.1 Predicted Throw and Penetration 91 A. 1.1 L i s t of Instruments 106 A. 5.1 Flow Rates Calculated with the P.W.A 115 B. 2.1 Combustion C a l c u l a t i o n from H.A. Simons Ltd 120 B.3.1 Flue Gas Composition 121 B. 3.2 Flow Rates i n the Prototype Furnace 125 C. 4.1 The E f f e c t of P a r t i c l e Size on Entrainment . . . . . . 151 Combustion and Carryover D. l Size Ranges and Moisture Contents of T y p i c a l 168 Components of Hog Fuel D.2 T y p i c a l Proximate Analysis of Moisture-Free . . . . . . 169 Wood Fuels D.3 T y p i c a l Ultimate Analysis of Moisture-Free 170 Samples of Hog Fuel Bark D. 4 T y p i c a l Heating Values f o r Moisture-Free 170 Bark and Wood E. 7 Variables Used i n Models of Predicted O v e r f i r e A i r . 224 J e t Centerline Speed and Trajectory v i i i LIST OF FIGURES Figure page 2.1 Schematic of a Hog Fuel B o i l e r 6 5.1 The Apparatus 25 5.1.1 The Model Overfire A i r Nozzles 27 5.2.1 Model A i r f l o w System 28 5.3.1 Pulsed Wire Anemometer Probe 30 5.3.2 Probe Traversing Mechanism 32 5.3.3 Coordinate System and Region of Measurement 33 with the P.W.A. 8.1.1 V e l o c i t y P r o f i l e , Case 4 49 X-1.6 f t , Z—4.46 f t 8.1.2 P r o f i l e of V e r t i c a l V e l o c i t y W, Cases 1, 2 and 4, . . 52 Z—4.46 f t , Y-8.1 f t 8.1.3 Case 1, V e l o c i t y i n the Plane Y-8.1 f t 53 8.1.4 Case 1, V e l o c i t y i n the Plane Y-2.5 f t 54 8.1.5 Flow Pattern Above the Grate 56 8.2.1 Smoke Flow Regions i n the Model 59 8.2.2 Case 2, Smoke i n the Plane Y-8 f t 60 8.2.3 Case 1, Smoke i n the Plane Y-8 f t 61 8.2.4 Case 2, Smoke i n the Plane X-5.4 f t 64 8.2.5 P r o f i l e of V e r t i c a l V e l o c i t y W, Cases 1 and 2, . . . . 67 Z-13.3 ft.Y-8.1 f t 8.2.6 V e l o c i t y P r o f i l e , Case 4, Z-1.63 f t , 68 X-7.3 f t 8.2.7 Average Turbulence Intensity v' 72 i n the Plane Y-8.1 f t 8.2.8 Isotropy Ratio R, Case 1 75 i n the Plane Y-8.1 f t v i i i i x 8.2.9 V e r t i c a l Turbulence Intensity w', Case 1 77 i n the Plane Y=8.1 f t 8.2.10 Average Turbulence Intensity v' 78 i n the Plane Z-.07 f t 8.2.11 Regions of Mixing i n the Furnace 82 8.3.1 J e t Trajectory and Centerline Speed, 84 Case 1, i n the Plane Y-8.1 f t 8.3.2 J e t Trajectory and Centerline Speed, . 85 Case 1, i n the Plane Y-14.4 f t B . l . l Present Overfire A i r Nozzle Configuration 118 B. 3.1 D i s t r i b u t i o n of Combustion A i r 123 C. 3.1 Schematic of Gas Temperature vs Oxygen 137 Concentration i n an Incinerator C.3.2 Lines of Equal Heating Value i n a Furnace 140 C.4.1 Combustion of a Hog Fuel P a r t i c l e 143 C.4.2 Schematic of the Apparatus of Pershing 145 C.5.1.1 Parameters near the Nozzle of a Buoyant 154 J e t i n Crossflow C.5.1.2 J e t i n Quiescent flow, No Buoyancy 155 C.5.1.3 J e t i n Uniform Crossflow 160 C.5.2.1 R e c i r c u l a t i o n Zones and Channeling 163 i n a Recovery B o i l e r C.5.3.1 Buoyancy i n a Furnace 164 E . l . l to E.1.19 Plots of V e l o c i t y Vectors 172 E.2.1 to E.2.7 V e l o c i t y P r o f i l e s , Y as Abcissa 191 E.3 V e r t i c a l V e l o c i t y W P r o f i l e s , X as Abcissa 199 E.4.1 to E.4.16 Plots of Turbulence Intensity 200 E.5.1 to E.5.2 Plots of Isotropy Ratio, R 216 E.6.1 to E.6.6 J e t Centerline Speed and Trajectory 218 ix LIST OF SYMBOLS a a c c e l e r a t i o n a speed of sound A, B c a l i b r a t i o n constants for p.w.a. A area ao angle of nozzle from h o r i z o n t a l Cm mean c e n t e r l i n e concentration at x Co mean concentration at nozzle e x i t C instantaneous f u e l gas concentration Cq quenching f u e l gas concentration Cs s t o i c h i o m e t r i c f u e l gas concentration C time averaged f u e l gas concentration c f l u c t u a t i n g component of f u e l gas concentration c' unmixedness f a c t o r Cp Cp'pressure c o e f f i c i e n t s do hydraulic diameter of nozzle Fd drag force per u n i t length of j e t Fb buoyant force per u n i t length of j e t F i i n e r t i a force Fv viscous force Fb bouyancy force Fp pressure force Fe e l a s t i c force Frd densimetric Froude number $ o v e r f i r e a i r mass flow rate/combustion a i r mass flow rate g a c c e l e r a t i o n due to gra v i t y x i : G.G.F. grate gas volume flow rate k s p e c i f i c heat r a t i o Lj j e t penetration l,V length I , t length scales of furnace and o v e r f i r e a i r nozzles s o L furnace load (steam mass flow rate) L length of s l o t m mass M r a t i o of dynamic pressures i n a j e t and crossflow Ma Mach number m* mass flow rate i n a j e t , at * mo mass flow rate at nozzle of a j e t Mi dynamic s i m i l a r i t y parameter M2 geometric s i m i l a r i t y parameter M 3 , M 3 ' kinematic s i m i l a r i t y paramaters M mean molecular weight Mo mean molecular weight of a i r Mf mean molecular weight of f l u e gas Mg mean molecular weight of gas r i s i n g from the grate D c combustion a i r mass flow rate mf f l u e gas mass flow rate mo o v e r f i r e a i r mass flow rate mg mass flow rate of gas r i s i n g from the grate H v i s c o s i t y N number of samples required v kinematic v i s c o s i t y O.F.A. o v e r f i r e a i r volume flow rate p absolute pressure Ap pressure drop r r a d i a l coordinate of j e t JJ i d e a l gas law constant Ri combustion a i r mass flow rate/steam mass flow rate R2 f l u e gas mass flow rate/steam mass flow rate Re Reynold's number p , p d e n s i t i e s Ap r e l a t i v e density P q gas density at o v e r f i r e a i r nozzle e x i t p density of gas r i s i n g from grate 8 p p a r t i c l e density p p density of hot gas stream h p density of c o l d gas c s speed s maximum speed m 5 scale of model T thickness of la r g e s t p a r t i c l e entrained T temperature t time of f l i g h t Th, Tc temperatures of hot and c o l d gas streams Th throw of a j e t e h a l f angle of j e t TJ mean v e l o c i t y component i n X d i r e c t i o n u' standard deviation of v e l o c i t y i n the X d i r e c t i o n u mean v e l o c i t y measured by p.w.a. u' standard deviation of v e l o c i t y measured by p.w.a. U.G.A. under grate a i r volume flow rate v, »' v e l o c i t y v' average turbulence i n t e n s i t y V volume v average v e r t i c a l v e l o c i t y at the grate, crossflow v e l o c i t y V q o v e r f i r e a i r nozzle v e l o c i t y vh v e l o c i t y of hot gas stream vh i n i t i a l v e l o c i t y of hot gas stream vm mean c e n t e r l i n e v e l o c i t y at * vo mean v e l o c i t y at nozzle e x i t W mean v e l o c i t y component i n Z d i r e c t i o n w'standard d e v i a t i o n of v e l o c i t y i n the Z d i r e c t i o n x, y. j e t model coordinates X, Y, Z coordinate axes of furnace y width of a s l o t % O.F.A. o v e r f i r e air/combustion a i r expressed as a percent X I V ACKNOWLEDGEMENT I wish to thank my advis or. Professor I.S. Gartshore f o r h i s guidance and assistance throughout my research and i n the preparation of t h i s t h e s i s . I also wish to thank my contact at H. A. Simons Ltd., Mr Douglas M. Bruce. Mr. Bruce acted as a second advisor, and h i s guidance and knowledge were indispensable i n my research, and i n the preparation of t h i s t h e s i s . I am g r a t e f u l f o r the f i n a n c i a l assistance f o r my research, provided by the Science Council of B r i t i s h Columbia under the G.R.E.A.T. assistance scheme and by H.A. Simons Ltd. I wish to express my appreciation to Dr. Terry Adams f o r h i s con s u l t a t i o n and l i t e r a t u r e , which provided most of the background f o r my research. I also wish to express my appreciation to Mr. D.H. Gray, Project Manager , B r i t i s h Columbia Forest Products Ltd., Crofton Pulp and Paper D i v i s i o n , f o r allowing me access to engineering data on the b o i l e r , and f o r f a c i l i t a t i n g v i s i t s to the m i l l . F i n a l l y , I would l i k e to thank Dave Camp f o r h i s work on the apparatus, the technicians i n the Mechanical Engineering shop f o r t h e i r advise and assistance while I was b u i l d i n g the model and the technicians i n the Instrumentation shop. page 1 1 Introduction At pulp and paper m i l l s i n B r i t i s h Columbia much energy i s produced by the combustion of hog f u e l . Hog f u e l consists of p a r t i c l e s of f o r e s t r y waste, inc l u d i n g wood and bark. Hog f u e l b o i l e r s are designed to minimize emissions of unburnt f u e l p a r t i c l e s , unburnt hydrocarbon gases, and carbon monoxide, to maximize e f f i c i e n c y and to achieve stable combustion over a range of loads. These objectives are met by s p e c i f y i n g the b o i l e r furnace geometry and the r a t i o of the o v e r f i r e a i r flow rate to the t o t a l combustion a i r flow rate, i n order to optimize the aerodynamic conditions i n the combustion zone. In hog f u e l b o i l e r s , the rate of consumption of hog f u e l i s l i m i t e d by carryover of f u e l p a r t i c l e s , and gaseous emissions. Non uniform v e l o c i t y p r o f i l e s , and r e c i r c u l a t i o n zones which cause channeling i n the flow r e s u l t i n increased carryover f o r a given rate of f u e l consumption. Overfire a i r j e t s are used to add oxygen to and mix gases i n the furnace combustion zone. Weak o v e r f i r e a i r j e t s cause channeling of the flow to occur as w e l l . Gas flow conditions i n a b o i l e r furnace are d i f f i c u l t to measure or observe because the gas temperatures in s i d e t y p i c a l l y range from 1000°F to 2000°F and viewing ports are us u a l l y too small and poorly positioned to see much i n the combustion zone. The flow f i e l d i s three dimensional and unsteady, and the geometry of the actual furnace i s complicated so that numerical modeling i s d i f f i c u l t . In a scale model with isothermal flow however, the complicated geometry of the furnace can be simulated and the flow page 2 f i e l d i n s i d e can be observed and measured. In November 1986, H.A. Simons Ltd. modified the o v e r f i r e a i r system of the #4 power b o i l e r at the Woodfibre pulp m i l l near Squamish B.C. Under the G.R.E.A.T scholarship program of the Science Council of B r i t i s h Columbia, i n a s s o c i a t i o n with H.A. Simons Ltd. , the e n t i r e flow f i e l d of t h i s hog f u e l furnace has been simulated with an isothermal scale model. A 1:15 scale p l a s t i c model was constructed of the modified b o i l e r . A i r at room temperature was blown through i t to simulate the gas flow f i e l d . Two sets of experiments were conducted at three r a t i o s of o v e r f i r e a i r to t o t a l combustion a i r ; the v e l o c i t y f i e l d was measured throughout the combustion zone with a pulsed wire anemometer, and with the i n j e c t i o n of smoke, over f i r e a i r j e t s , v e r t i c a l s t r a t i f i c a t i o n and mixing of gases were observed. Results of v i s u a l i z a t i o n of the flow f i e l d and measurement of v e l o c i t i e s i n the scale model, together with a l i t e r a t u r e review of combustion i n hog f u e l b o i l e r s provide p o t e n t i a l l y useful information about aerodynamics and combustion i n hog f u e l furnaces. The r e s u l t s obtained w i l l be u s e f u l f or the design of b o i l e r s or b o i l e r modifications i n the future, and the experiments demonstrate the use of scale models as a design t o o l . In the present t h e s i s , hog f u e l b o i l e r s and combustion i n hog f u e l b o i l e r s are b r i e f l y summarised i n sections 2 and 3. Isothermal modeling i s discussed i n s e c t i o n 4. Experimental apparatus, procedures r e s u l t s and discussion are i n the subsequent page 3 sections. The appendices, which may be of i n t e r e s t to some readers but are not necessary f o r an understanding of hog f u e l combustion and i t s r e l a t i o n to the present i n v e s t i g a t i o n , contain d e t a i l s of the experiments, and the l i t e r a t u r e review. page 4 2 Basic Elements of a Hog Fuel B o i l e r This scale model study i s concerned with a hog f u e l b o i l e r furnace. A hog f u e l b o i l e r however, consists of elements other than the furnace and they must be considered as w e l l . The furnace i s the space where f u e l and a i r are brought together and burned. The combustion zone i s the area of the furnace where combustion a c t u a l l y takes place. Figure 2.1 i s a schematic of a t y p i c a l large i n d u s t r i a l spreader stoker b o i l e r . A i r i s supplied to the furnace by the combustion a i r system through the under grate a i r plenum and through o v e r f i r e a i r j e t s . Pressure i s provided by the forced d r a f t fan. In the combustion a i r preheater the a i r i s preheated by heat exchange with f l u e gas. At the bottom of the furnace i s the f u e l grate and under grate plenum. Pre heated combustion a i r enters the under grate plenum from the combustion a i r system and enters the furnace through holes or spaces i n the f u e l grate. Fuel can enter the furnace and be d i s t r i b u t e d onto the f u e l grate by various means. In the furnace shown i n the schematic, f u e l i s f e d i n through hoppers and a i r i s blown i n through the windswept spouts to spread the f u e l evenly over the grate. Ash i s removed from the grate continuously or, i n some models, p e r i o d i c a l l y . Hog f u e l b o i l e r s have one or more h o r i z o n t a l rows of o v e r f i r e a i r nozzles i n the walls through which part of thecombustion a i r i s supplied. Jets of a i r issue from these nozzles into the combustion zone to provide a i r f o r the combustion of gases and page 5 entrained particles and to promote mixing of gases. Most hog fuel boilers have auxiliary o i l or natural gas fired burners. They are used to supplement the power output of hog fuel when demand for steam is high, or when the hog fuel feed is stopped for grate cleaning. When the flame temperature resulting from the combustion of hog fuel alone is too low, which can occur i f the fuel is very wet, blackouts can occur. Reignition (called "puffing") can cause dangerous pressure transients to occur. Burners are used to prevent blackouts and puffing. Burners are also used to respond to quick variations in steam demand. page 6 Figure 2 . 1 , Schematic of a Hog Fuel B o i l e r The gaseous combustion products that e x i t the combustion zone are c a l l e d f l u e gas. Heat i s extracted from the f l u e gas by the water cooled furnace walls ( c a l l e d "water walls") and tube banks. The f l u e gas passes through the combustion a i r preheater and enters a p a r t i c u l a t e removal device such as a multicyclone. The induced draught fan keeps the furnace at a s l i g h t vacuum and exhausts the f l u e gas to the stack. page 7 3 Results of Survey of Theory of Combustion i n Hog Fuel B o i l e r s A survey of the theory of combustion of hog f u e l b o i l e r s i s i n Appendix C of t h i s t h e s i s . Results and general c r i t e r i a f o r good combustion i n hog f u e l b o i l e r s from the survey that are relevant to the scale model study, and issues that are addressed with the scale model study, are presented i n t h i s section. The r e s u l t s are divided into four categories: a) combustion of f u e l on the grate, <ii> gaseous combustion, (iii) combustion of entrained f u e l p a r t i c l e s and (iv) aerodynamics. Aerodynamics includes mathematical models of o v e r f i r e a i r j e t s , channeling, r e c i r c u l a t i o n zones, and buoyancy e f f e c t s . ( i) Combustion of Fuel on the Grate Gas v e l o c i t i e s should be kept as low as p o s s i b l e , through and above the grate to minimize the entrainment of p a r t i c l e s of f u e l and ash. The under grate a i r flow rate, and therefore, gas v e l o c i t i e s above the grate can be minimized only i f the a i r to f u e l r a t i o i s uniform across the f u e l grate. This i s l i k e l y to be achieved only i f the f u e l p i l e has an even thickness over a l l of the f u e l grate and the v e l o c i t y p r o f i l e above the grate i s uniform. For i n c r e a s i n g l y wet f u e l , the under grate a i r flow rate must be increased to maintain stable combustion i n the f u e l p i l e . Radiation of heat to the f u e l p i l e from gas phase combustion i n the o v e r f i r e a i r j e t s , a u x i l i a r y burners or furnace arches dries the f u e l , minimizing the page 8 under grate a i r flow required. V e l o c i t i e s above the grate, at three r a t i o s of the o v e r f i r e a i r flow rate (O.F.A.) to the t o t a l combustion a i r flow rate were measured i n the scale model, ( i i ) Gaseous Combustion Combustion occurs almost instantaneously as combustible gases and oxygen are mixed. Therefore, combustion of gases i s l i m i t e d by mixing and not by the vrate of chemical reaction. O v e r f i r e a i r j e t s produce uniform time averaged p r o f i l e s of gas concentration and temperature i n the combustion zone (macro mixing). The a i r to f u e l r a t i o may be instantaneously l e s s than stoichiometric however, r e s u l t i n g i n the emission of unburnt f u e l gases, mainly carbon monoxide. Intense small scale turbulence mixes gases on a micro scale, breaking up packets of non uniform gas concentration, thus reducing instantaneous deviations of gas concentration from the mean and reducing emissions. O v e r f i r e a i r j e t s reduce the residence time required f o r complete combustion by mixing the gases, thus reducing emissions. Combustion can be concentrated j u s t above the f u e l bed on the grate by the mixing e f f e c t of the o v e r f i r e a i r j e t s . page 9 Combustion i n the f u e l p i l e provides an i g n i t i o n source f o r gas combustion above the f u e l p i l e , s t a b i l i z i n g i t . The e f f e c t of o v e r f i r e a i r j e t s on macro scale mixing of gases i s demonstrated i n the model with smoke flow v i s u a l i z a t i o n . The e f f e c t of o v e r f i r e a i r j e t s on micro scale mixing i s addressed with turbulence i n t e n s i t y measurements i n the model, ( i i i ) Combustion of Entrained Fuel P a r t i c l e s Fuel p a r t i c l e s do not change i n s i z e or shape as they dry and devolatize, however the mass of f u e l p a r t i c l e s i s reduced by 85% to 90% (dry b a s i s ) . P a r t i c l e s that f a l l onto the grate are p a r t i a l l y or completely dried, so that they can enter the furnace above the o v e r f i r e a i r nozzles and f a l l to the grate. P a r t i c l e s that reach the grate however, r e t a i n t h e i r o r i g i n a l s i z e and shape and lose up to 90% of t h e i r dry mass as they d e v o l i t i z e . Therefore, v e l o c i t i e s above the grate must be kept low to prevent re-entrainment of p a r t i c l e s from the grate. Carryover decreases as the r a t i o of o v e r f i r e a i r to under grate a i r increases because entrainment from the grate i s reduced as the under grate a i r flow rate i s reduced. ( i v) Aerodynamics Bouyancy e f f e c t s need not be considered i n o v e r f i r e a i r j e t models i n hog f u e l b o i l e r s . Mathematical models f o r unconfined j e t s may not be page 10 adequate when applied to opposing banks of j e t s i n hog f u e l furnaces. Ove r f i r e a i r j e t s have been observed to cause large r e c i r c u l a t i o n zones near the walls, and a region of high v e r t i c a l v e l o c i t y near the middle of b o i l e r s , with l i t t l e mixing. Non uniform v e r t i c a l flow with l i t t l e mixing (channeling), probably r e s u l t s In increased carryover and gaseous emissions. Buoyancy e f f e c t s , r e s u l t i n g i n channeling of the flow, can occur i f there are h o r i z o n t a l temperature gradients i n the flow. Horizontal temperature gradients are caused by leakage of outside a i r into the furnace (tramp a i r ) , uneven combustion on the f u e l grate, or channeling i n the flow. Models f o r unconfined j e t s , presented i n Appendix C . 5 , are tested under conditions s i m i l a r to those encountered i n a hog f u e l b o i l e r , i n the present scale model study. R e c i r c u l a t i o n zones and channeling are addressed with the r e s u l t s of the scale model study. Problems with channeling caused by buoyancy are i d e n t i f i e d with the r e s u l t s of the scale model study. General C r i t e r i a f o r Good Combustion i n Hog Fuel B o i l e r s (i) The undergrate a i r flow rate should be j u s t high enough to maintain stable combustion i n the f u e l p i l e . The r e s t of the page 11 combustion a i r should be provided by o v e r f i r e a i r j e t s so that v e l o c i t i e s above the f u e l grate, and entrainment of devolatized f u e l p a r t i c l e s can be minimized. (ii) The v e l o c i t y p r o f i l e above the grate should be uniform. V a r i a t i o n s from the mean v e r t i c a l v e l o c i t y above the grate r e s u l t i n non uniform a i r to f u e l r a t i o s and rates of combustion of f u e l on the grate. Regions of high v e r t i c a l v e l o c i t y at the grate r e s u l t i n increased p a r t i c l e entrainment from the f u e l bed. ( i i i ) P r o f i l e s of gas concentration, and temperature i n h o r i z o n t a l cross sections of the furnace should be uniform to prevent regions of excessively high or low a i r to f u e l r a t i o , and h o r i z o n t a l temperature gradients, that can cause channeling due to buoyancy e f f e c t s , from occuring. (iv) O v e r f i r e a i r j e t s should be arranged to mix o v e r f i r e a i r and furnace gases close to the grate. Mixture of combustible gases and a i r near the grate concentrates the gaseous combustion near the grate which helps to dry the f u e l p i l e . Residence times of gas or p a r t i c l e s entrained into regions of high v e r t i c a l v e l o c i t y are increased by intense mixing i n the combustion zone. page 12 4 Modeling of Furnace Flow F i e l d s Isothermal modeling has been used many times to simulate the gas flow f i e l d i n furnaces, ducts and combustors. With isothermal models, q u a l i t a t i v e and quantitative observations can be made of v e l o c i t y f i e l d s , mixing of gases, residence times and the t r a j e c t o r i e s of entrained p a r t i c l e s . In s e c t i o n 4.1, a n a l y t i c a l , numerical and scale model methods are reviewed. In section 4.2, non dimensional parameters f o r scale modeling are derived and parameters that have l i t t l e relevance are eliminated. A i r flow rates i n the model and the s i z e of the o v e r f i r e a i r nozzles i n the model must be set to produce a flow f i e l d i n the model that i s homologous to the flow f i e l d i n the prototype furnace. In se c t i o n 4.3, procedures f o r c a l c u l a t i n g the a i r flow rates i n the model, and the s i z e of the o v e r f i r e a i r nozzles i n the model are derived. Procedures to c a l c u l a t e s c a l i n g f a c tors are derived i n se c t i o n 4.3. The s c a l i n g factors are needed to scale lengths and v e l o c i t i e s measured i n the model flow f i e l d to represent the actual corresponding quantities i n the furnace. 4.1 Review of Methods In t h i s s e c t i o n , a n a l y t i c a l , numerical and scale model methods of modeling furnace gas flow f i e l d s are reviewed and modeling studies are c i t e d . A n a l y t i c a l Methods A n a l y t i c models f o r j e t s In unconfined, uniform crossflow and buoyancy e f f e c t s are discussed i n Appendix C.5. Lamb et a l (1) page 13 have used the models i n a study of a refuse i n c i n e r a t o r . A n a l y t i c a l models however, can only model s p e c i f i c elements of the flow such as unconfined j e t s under hi g h l y i d e a l i z e d conditions and can not accurately model the i n t e r a c t i o n of elements of the flow such as the e f f e c t of walls and opposing j e t s . Numerical Methods Numerical methods involve d i s c r e t i z i n g the governing d i f f e r e n t i a l equations and s o l v i n g them with a computer. Numerical methods include f i n i t e d ifference and f i n i t e element methods. The gas flow f i e l d i n a furnace i s unsteady, h i g h l y turbulent and three dimensional, and the boundary conditions involve complicated geometry. Boundary layers, free shear layers around j e t s , and r e c i r c u l a t i o n zones a l l e x i s t w i t h i n the flow f i e l d so that the accuracy of turbulence modeling i s uncertain. Numerical models have the advantage, however, that chemical reaction, heat t r a n s f e r and mass tr a n s f e r can a l l be modeled simultaneously with the flow f i e l d . K h a l i l (2) has summarized some furnace modeling studies. The geometries are a l l simple; the furnace i s modeled as a duct with a square or round cross s e c t i o n and a burner at one end. There are no o v e r f i r e a i r j e t s , grates, bullnoses or arches. V e l o c i t i e s , temperatures and gas concentrations were ca l c u l a t e d . Comparison of the r e s u l t s with measurements made i n furnaces revealed that temperature r e s u l t s were accurate within 10% except i n r e a c t i o n zones where they were within 20%. V e l o c i t y f i e l d s were q u a l i t a t i v e l y correct. The existence of r e c i r c u l a t i o n zones was page 14 c o r r e c t l y predicted, but t h e i r s i z e was not. Scale Model Methods Scale models have often been used to model gas flows i n furnaces and combustors (2,3,4,5,6,7,8). Flow patterns i n the furnace are simulated by pumping or blowing l i q u i d or gas through a scale model of the furnace. V e l o c i t i e s , mixture of gases, residence times and p a r t i c l e t r a j e c t o r i e s can be measured d i r e c t l y and scaled to the prototype, and flow patterns can be v i s u a l i z e d . Combustion i s u s u a l l y not simulated i n scale models because i t complicates the construction of the models, and because i t makes v e l o c i t y measurements and flow v i s u a l i z a t i o n d i f f i c u l t . Therefore, a i r or water at room temperature i s u s u a l l y used i n the models to simulate the flow f i e l d . The use of isothermal f l u i d i n scale models i s c a l l e d isothermal modeling. Isothermal modeling i s a r e l a t i v e l y quick way to model furnace flows. Complicated geometries can be produced and gas flow rates i n the model can be changed r e a d i l y . Thus the e f f e c t s of changes i n furnace geometry and o v e r f i r e and undergrate gas flow rates can be observed or measured. Measurements and observations f a l l into two categories: flow v i s u a l i z a t i o n and d i r e c t measurement. Flow v i s u a l i z a t i o n involves making flow patterns v i s i b l e so that they can be observed, photographed, video taped or filmed. V e l o c i t i e s can be measured d i r e c t l y with p i t o t tubes, hot wires, l a s e r doppler anemometers or other devices f o r measuring f l u i d v e l o c i t y . Mixture and residence time of gases can be q u a n t i f i e d by measuring the concentration of page 15 tra c e r f l u i d s introduced into the flow. Furnace flow f i e l d s u s u a l l y contain r e c i r c u l a t i o n zones, regions of reversing flow and regions where the turbulence i n t e n s i t y i s large compared to the mean v e l o c i t y . P i t o t tubes and hot wires therefore, can not be used accurately throughout the flow f i e l d . Some examples of isothermal model studies of furnaces can be found i n the l i t e r a t u r e (3,4,5,6). Topley (6) optimized the placement angle and s i z e of o v e r f i r e a i r nozzles, and the placement of arches i n a hog f u e l furnace with experiments conducted on a scale model. For various configurations, flow patterns were observed with dust, and v e l o c i t y p r o f i l e s were measured. When the flow patterns were found to be acceptable, the undergrate a i r was heated to simulate combustion, and temperature and v e l o c i t y p r o f i l e s were then measured. The p i t f a l l of isothermal modeling i s that only p a r t i a l modeling i s po s s i b l e . One approximation that must be made i s that combustion and heat t r a n s f e r are not modeled. Aerodynamics and mixing of gases can be modeled, but assumptions must be made about how combustion and heat trans f e r a f f e c t gas density, since the density a f f e c t s the aerodynamics. Since there i s combustion in s i d e a furnace, the temperature of combustion a i r entering i t i s much lower than the temperature i n s i d e , with a r e s u l t i n g d i f f e r e n c e i n density. Another approximation r e s u l t s therefore, because the f l u i d i n the model has a constant density. Since there i s a density d i f f e r e n c e i n the page 16 prototype furnace, and not i n the model, i t i s not pos s i b l e to have dynamic s i m i l a r i t y , kinematic s i m i l a r i t y and geometric s i m i l a r i t y simultaneously, between the model and prototype furnace flow f i e l d s . I f the l i m i t a t i o n s of isothermal modeling applied to the s p e c i f i c a p p l i c a t i o n are well understood by the experimenter, so that areas where modeling i s not accurate are known, correct q u a l i t a t i v e p r e d i c t i o n s and reasonable quantitative predictions can be obtained. Isothermal modeling of furnaces however, i s not an exact science. 4.2 I d e n t i f i c a t i o n of Non Dimensional Parameters Relevant f o r Isothermal Modeling S i m i l a r i t y between model and prototype implies that the r a t i o s of a l l l i k e q u antities i n the prototype are the same as the r a t i o s of s i m i l a r quantities i n the model. For example, the r a t i o of any two forces, any two mass d i f f u s i o n rates, or any two lengths must be the same i n the prototype as i n the model. The r a t i o s of l i k e q u antities are represented by non dimensional parameters. In order to have s i m i l a r i t y , these non dimensional parameters must be equal i n the model and the prototype. In the following sections the subscript "p" r e f e r s to the prototype furnace and the subscript "m" r e f e r s to the model. The subscript "g" r e f e r s to gases r i s i n g from the grate, "o" r e f e r s to o v e r f i r e a i r at the point of introduction to the furnace and " f " r e f e r s to the f l u e gas at the furnace e x i t . The forces i n the flow f i e l d can be represented by the page 17 following c h a r a c t e r i s t i c forces: "A " means " i s dimensiorially equivalent to", I n e r t i a Force=Fi^ ma ^ pl3(— ) ^  pt3v2 t2 T T - _ A dv , 2 A • Viscous Force=Fv- A* T# * " A*v* ot Buoyancy Force=Fb^ (p -p )&V ^  Apt 3g Pressure Force«=Fp^ ApA ^Ap£ 2 E l a s t i c Force-Fe- pA * pXTl3 a 2-k*T I f k i s the same i n the prototype and model a2* «r Therefore, Fe^ pa 2£ 2 A l l v a r i a b l e s l i s t e d below are c h a r a c t e r i s t i c dimensions: m—mass o-acceleration 6=time t-length v-^velocity V=volume A=area M - v i s c o s i t y g=acceleration due to gra v i t y p ,p - d e n s i t i e s page 18 Ap=p -p p=pressure Ap=pressure drop a=the speed of sound k=the s p e c i f i c heat r a t i o Sl-ideal gas law constant The non dimensional parameters are: F i pf/V Re—Reynolds Number==— —*-— J Fv fx F i 1 / 2 1 / 2 v Frd-Densimetric Froude Number-f — 1 -f—7—1 —— L A ' J (tg) 1' 2 Cp-Pressure C o e f f i c i e n t - 2x=^ - , .„ 2 F i 1/2 pv Ma-Mach number-f ^ - 1 - — In a d d i t i o n to c h a r a c t e r i s t i c force r a t i o s , c h a r a c t e r i s t i c r a t i o s representing i n e r t i a , geometry, v e l o c i t y , mass balances, energy balances and chemical r e a c t i o n must be the same i n the model as w e l l as the prototype i n order to have s i m i l a r i t y . This i s r a r e l y possible i n model studies; and i s c l e a r l y impossible i n an isothermal model study because there i s no heat tr a n s f e r , mass tr a n s f e r or chemical reaction i n isothermal models. I t i s u s u a l l y possible to neglect some non dimensional parameters i n a modeling study. This i s c a l l e d p a r t i a l modeling. Spalding (7) summarized which parameters can be Ignored, and under what conditions i n scale model studies of furnaces and combustors. page 19 Some of h i s points are l i s t e d here: I f Ma <.8 at the point of highest v e l o c i t y i n the prototype,then Ma need not equal Ma . p m I f Re and Re < 10000 when ca l c u l a t e d with the l a r g e s t m p length and v e l o c i t y In the problem, then Re need not m equal Re , and molecular d i f f u s i o n parameters can be p ignored. Combustion a f f e c t s the flow f i e l d forces p r i m a r i l y by a f f e c t i n g the density. The density f i e l d a f f e c t s the i n e r t i a and buoyancy forces i n the flow f i e l d . The model has a uniform density f i e l d while the furnace does not. Since a i r at room temperature i s the only medium used i n the model, buoyancy e f f e c t s are ignored i n the model. In Appendix C.5 i t i s demonstrated that buoyancy Is not important f o r j e t s i n unconfined, uniform crossflow. The case of unconfined j e t s may not be s u f f i c i e n t l y s i m i l a r to the case of opposing j e t s that occur i n a hog f u e l furnace, however. The i n e r t i a l e f f e c t s of the density f i e l d can be modeled i f the gas above the grate i s assumed to have a uniform density, and the o v e r f i r e a i r i s assumed to enter the furnace at a d i f f e r e n t uniform density, i n the prototype furnace. The relevant parameter i s : _ , 2 2 Fi p I V Mi» o - 0 0 o Fi , 2 2 g p I V 8 8 8 Mi-ratio of o v e r f i r e a i r i n e r t i a to furnace gas i n e r t i a page 20 p~overfire a i r density p =gas density above the grate s t Q=overfire a i r nozzle length scale t -furnace length scale 8 v - o v e r f i r e a i r c h a r a c t e r i s t i c v e l o c i t y o J v - c h a r a c t e r i s t i c v e l o c i t y of the gases above the grate 8 The relevant dynamic parameters then are Mi and Cp. Thus dynamic s i m i l a r i t y implies that Mim-Mip and Cpm-Cpp. Geometric and kinematic s i m i l a r i t y are also required. They w i l l be discussed i n s e c t i o n 4.3. 4.3 Derivation of Scaling Laws for the Isothermal Model In t h i s section, rules are derived according to which c o n t r o l l e d v a r i a b l e s i n the model are c a l c u l a t e d so that the flow f i e l d i n the model i s homologous to the flow f i e l d i n the prototype furnace, at the s p e c i f i e d furnace operating condition. Rules are also derived to estimate q u a n t i t i e s i n the prototype furnace from q u a n t i t i e s measured i n the model. Controlled Variables i n the Model In the model, v a r i a b l e s that can be c o n t r o l l e d are the o v e r f i r e a i r volume flow rate (O.F.A)., the volume flow rate of gas r i s i n g from the grate (G.G.F.) and geometry. They must be set appropriately i n order to simulate s p e c i f i c operating conditions i n the prototype furnace. Since combustion, and the r e s u l t i n g e f f e c t s on chemical composition of gases and temperature are approximated by assuming that gases from the grate and over f i r e a i r enter the furnace at page 21 two uniform d e n s i t i e s , s i m i l i t u d e s of mass balance, energy balance and combustion k i n e t i c s are ignored. Also, the e f f e c t s of compr e s s i b i l i t y , v i s c o s i t y and buoyancy are ignored. Therefore, the remaining requirements f o r s i m i l a r i t y are s i m i l a r i t y of i n e r t i a forces, geometric s i m i l a r i t y and kinematic s i m i l a r i t y . The parameter f o r i n e r t i a forces (from s e c t i o n 4.2): The parameter f or geometry : I M2 4.3.2 U For v e l o c i t y : v Ma 4.3.3 v 8 Cp i s a dependent parameter i n t h i s case, and therefore, w i l l be the same i n the model and prototype i f the c o n t r o l l a b l e parameters are equal i n the model and the prototype. Therefore f o r s i m i l a r i t y , the following conditions must be met: Mi- F i F i 4.3.1 8 8 8 Ml -Ml m P M2 -M2 m P M3 - M 3 m p In the model — -1, therefore, i f Mi -Mi : p m p p V Q 2 tQ 2 V Q 2 lQ 2 t T \( V~ \ ( T \ " ( v ~ ) m C €~ 8 8 8 8 8 4.3.4 page 22 In t h i s case e i t h e r M2 *M2 , or M3 * M 3 since: m p m p P P v p J p v p -'m 8 8 I t i s possible to enlarge the o v e r f i r e a i r nozzles i n the model, to compensate f or the r a t i o of de n s i t i e s i n the prototype furnace, thereby s a c r i f i c i n g geometric s i m i l a r i t y of c o n t r o l l e d v a r i a b l e s somewhat while maintaining kinematic s i m i l a r i t y of c o n t r o l l e d v a r i a b l e s , as follows: Let M3 - M 3 , therefore: n> p V Jm V •> \ _0 T 8 8 Substitute 4.3.5 into 4.3.4 to get 4.3.5 I I p 1/2 ( r ) . - ( r ) . ( r ) . p <- p •> p 8 8 4.3.6 This i s c a l l e d the Thring Newby c r i t e r i o n . I t has been used many times i n furnace and burner models (4,5,6,7,9,10). The o v e r f i r e a i r volume flow rate, O.F.A.— vlZ, and the 0 0 volume flow rate of gases r i s i n g from the grate, G.G.F.— v ( 2 . 8 8 From equation 4.3.6: I 2n I 2 4.3.7 Rearrange 4.3.7 F A ^ 2 ^ 0 2 ^G.G.F.-'in " ^ . C F . - ' p ^ T ' ^ p ^T~^m 0 8 4.3.7a Substitute 4.3.6 into 4.3.7a page 23 ^ G.G.F.^m " ( G.G.F.^p ^ p ^ P 4.3.8 s A s p e c i f i c dimension of the o v e r f i r e a i r nozzles i s the diameter do . Therefore from 4.3.6 do tg p 1/2 m m r 0 -v do tg ^ p -'p P P 8 and since the scale of the model, S — -=—— . p do p 1/2 TZT - S C — ) 4-3.9 do ^ P  J v p 8 Equation 4.3.5 implies kinematic s i m i l a r i t y and equation 4.3.8 implies dynamic s i m i l a r i t y of the c o n t r o l l e d v a r i a b l e s , and equation 4.3.9 implies s i m i l a r i t y of mass flow rates. Davidson (10) analyzed the Thring Newby c r i t e r i o n and concluded that i t provides a good simulation away from the furnace walls. I t does not model precombustion zones i n the j e t s , and a progressive error i s introduced as burners become c l o s e r together. In the present study, the model geometry i s designed according to equation 4.3.9 , and the model a i r flow rates are set according to equation 4.3.8 Estimation of Parameters i n the Prototype Furnace from Parameters Measured i n the Model Since kinematic s i m i l a r i t y i s assumed to occur between the model and the prototype furnace when the a i r flow rates and geometry are set according to equations 4.3.8 and 4.3.9, M3 can be page 24 generalized as: v' M 3 ' — -V 8 v'=any v e l o c i t y i n the flow f i e l d , and therefore: M3' - M 3 ' 4.3.10 p m G.G.F. £g Since v — , 2 , and S — - j — , equation 4.3.10 can be 8 i> V { 8 P rearranged as: v'~ v' C§44^ 3 S 2 4.3.11 Equation 4.3.11 i s used to scale v e l o c i t i e s measured i n the model to the prototype furnace. At a distance of at l e a s t eight nozzle diameters from the over f i r e a i r nozzles, geometric s i m i l a r i t y i s approached ( 8 ) . Therefore, i t i s v a l i d to scale lengths i n the model flow f i e l d as follows: I ' - S" 1* ' 4.3.12 p m where t ' i s any length measured i n the model flow f i e l d . Pressure drops, measured i n the model, can be scaled by assuming that Cp'= Cp' , where Cp' i s any pressure drop i n the m p flow f i e l d . page 25 5 The Apparatus The apparatus consists of a p l e x i g l a s s model of the i n t e r i o r of the furnace, a i r flow systems, and equipment f o r v e l o c i t y measurement with a pulsed wire anemometer (p.w.a.) and flow v i s u a l i z a t i o n . A photograph of the apparatus appears i n figure 5 . 1 . The apparatus i s described i n the following sections. A l i s t of equipment i s i n Appendix A. Information about the prototype furnace i s i n Appendix B. Figure 5 .1 The Apparatus page 26 5.1 The Model The model was b u i l t to a scale of 1:15. I t i s 43 inches high with an i n t e r i o r cross section 13.6 inches wide by 14 inches deep. The model includes the furnace, the o v e r f i r e a i r nozzles and the undergrate plenum. The furnace i s simulated from the f u e l grate to the e x i t , at the v e r t i c a l plane between the cent e r l i n e s of the mud drum and the steam drum. The e x i t i s shown i n f i g u r e 5.3.3. The model does not include the o i l burners, f u e l hoppers, access doors, viewing ports or the prot r u s i o n of the water w a l l tubes in s i d e the furnace. The superheater and evaporator tube banks are simulated by a screen with approximately the same pressure los s c o e f f i c i e n t i n the furnace e x i t plane. There are two types of o v e r f i r e a i r nozzles i n the prototype furnace as shown In f i g u r e B . l . l . One type of nozzle i s h o r i z o n t a l and mounted f l u s h to the furnace side wa l l ; the other type has an end, angled down 15 degrees from h o r i z o n t a l that protrudes 6 inches i n t o the furnace. The o v e r f i r e a i r nozzles i n the model are enlarged according to the Thring Newby c r i t e r i o n . S-l/15. From Appendix B: p-1.168xl0" 3 s l u g / f t 3 and p=5.373xl(f 4 s l u g / f t 3 , and therefore 0 8 from equation 4.3.9, = 1/10.2. The nozzles are shown i n figure d o p 5.1.1. page 27 Figure 5.1.1 The Model Overfire A i r Nozzles The f u e l grate i s made of pegboard with 1/8 inch diameter holes on one inch centers. The r a t i o of open area to grate area i s approximately the same i n the model as i t i s i n the prototype furnace. The undergrate plenum includes two s t r i p s of 1/4 Inch thick p l a s t i c , underneath the grate, to simulate I beams i n the prototype. Between the furnace e x i t and the economizer tube banks, the gas flow i s d i r e c t e d downwards by a b a f f l e , which i s simulated by a b a f f l e i n the exhaust duct of the model, near the e x i t of the model. 5.2 A i r Flow System The a i r flow system i s shown schematically i n f i g u r e 5.2.1. page 28 I t consists of the o v e r f i r e a i r system and the undergrate a i r system. O v e r f i r e A i r P l e n u n U n d e r G r a t e P l e n u n E x h a u s t n B l o w e r ( s p e e d c o n t r o l l e d b y v a r i a b l e t r a n s f o r m e r ) U N D E R G R A T E AIR S Y S T E M - Z p -O r t f l c e P l a t e s Y C o n t r o l ' s / A V a l v e s A O v e r f i r e A i r I n l e t V Q Conpr«««>p R e g u l a t o r • V E R F I R E AIR U n d e r G r a t e A i r I n l e t S Y S T E M Figure 5.2.1 Model A i r f l o w System Ov e r f i r e A i r System The flow to the o v e r f i r e a i r nozzles i s driven by a large compressor. A regulator controls the pressure i n two s i m i l a r systems, one f o r the o v e r f i r e a i r nozzles on each side of the model. In each system the flow rate i s set with a valve. The pressure drop across an o r i f i c e plate i s measured with a manometer. The pressure drop i s c a l i b r a t e d to flow rate. D e t a i l s of the c a l i b r a t i o n are i n Appendix A. The a i r i s d i s t r i b u t e d to a l l of the nozzles on each side of page 29 the model by a large plenum. A flow straightener produces a uniform v e l o c i t y p r o f i l e upstream of the nozzles. The center l i n e v e l o c i t y i n each h o r i z o n t a l nozzle was measured with a p i t o t tube and found to be the same i n each, within T5%. Under Grate A i r System The e x i t of the model Is connected by a 12 inch diameter duct to the suction side of a blower. The model i s , therefore, under a s l i g h t vacuum. The under grate a i r flow rate i s c o n t r o l l e d by adjusting the blower motor speed with a v a r i a b l e transformer. A f i v e foot length of two inch I.D. p l a s t i c pipe i s connected to the a i r i n l e t of the under grate plenum by a pipe elbow so that the pipe i s as close to v e r t i c a l as the 12 inch diameter duct w i l l allow (about 20° from v e r t i c a l ) . The G.G.F. flow rate i s measured by measuring the pressure drop across an o r i f i c e p l a t e i n the pipe with an i n c l i n e d manometer. The pressure drop i s c a l i b r a t e d to flow rate, as shown i n Appendix A. During some tes t s In which low O.F.A. flow rates were used, the a i r flow was found to be unsymmetrical about the plane of symmetry i n the model due to the 20° i n c l i n a t i o n of the pipe. A flow straightener c o n s i s t i n g of v e r t i c a l s t r i p s of sheet metal was i n s t a l l e d i n the pipe bend to correct the problem f o r these t e s t s . 5.3 Pulsed Wire Anemometer Equipment The pulsed wire anemometer (p.w.a.) can measure the magnitude and sense of one v e l o c i t y component i n reversing flows and h i g h l y turbulent flows. When many measurements are made, the mean v e l o c i t y , u, and the standard deviation of v e l o c i t y , u', can be page 30 calc u l a t e d . The equipment consists of a probe, a c o n t r o l l i n g u n i t , a 32K personal computer, a disk drive, a p r i n t e r and an o s c i l l i s c o p e . A program i n the computer operates the c o n t r o l l i n g u n i t , and ca l c u l a t e s the mean and standard d e v i a t i o n of v e l o c i t y with i t s output. The r e s u l t s are p r i n t e d out with the p r i n t e r . The o s c i l l i s c o p e i s used to observe the signals produced by the probe and the c o n t r o l l i n g u n i t . ^ The probe i s i l l u s t r a t e d schematically i n fi g u r e 5.3.1. A pulse of current i s sent to the pulse wire f o r a period of up to 16 microseconds and a 1 Mhz d i g i t a l counter i s started. The energy d i s s i p a t e d i n the pulse wire heats up a ribbon of a i r as i t flows past the wire. When the ribbon of warm a i r comes i n contact with one of the sensor wires, a s i g n a l i s produced i n the wire, t r i g g e r i n g the counter to stop. I f the s i g n a l i s produced i n the Pulsed Wire Sensor Vires Suppor-t For Vires Figure 5.3.1 Pulsed Wire Anemometer Probe page 31 p o s i t i v e wire, a p o s i t i v e v e l o c i t y i s indicated, and i f the s i g n a l i s produced i n the negative wire, a negative v e l o c i t y i s indicated. The number In the counter, when i t stops, i s the time of f l i g h t of the heated a i r i n microseconds, and i t i s c a l i b r a t e d against v e l o c i t y . D e t a i l s of the c a l i b r a t i o n are i n Appendix A. The probe can measure one component of v e l o c i t y only. However, i f the probe i s rotated by 90° about the axis of Its shaft, the perpendicular v e l o c i t y component can be measured as w e l l . Many measurements are made of one v e l o c i t y component, at a p a r t i c u l a r traverse l o c a t i o n , and the computer c a l c u l a t e s the mean v e l o c i t y and the standard deviation of v e l o c i t y . The probe i s held steady at any desired traverse l o c a t i o n with the t r a v e r s i n g mechanism shown i n f i g u r e 5.3.2. Figure 5.3.2 shows d e t a i l s of the mechanism. Figure 5.3.3 shows the region that can be traversed i n prototype furnace u n i t s , and the coordinate axes. There i s a s e r i e s of v e r t i c a l s l o t s i n the model rear w a l l . The p o s i t i o n of the probe i n the X-Z plane i s determined by the p o s i t i o n of the probe holder i n the s l o t s . The s l o t s are f i l l e d with foam rubber s t r i p s and sealed with ducting tape. The probe holder consists of a 1/4 inch diameter shaft and the shaft holder. The probe i s mounted on the end of the shaft. The Y ordinate of the probe can be adjusted by moving the shaft a x i a l l y i n the shaft holder. The shaft holder can be clamped to the model wa l l , and the shaft can be clamped wit h i n the shaft holder. page 32 Figure 5.3.2 Probe Traversing Mechanism Two mean v e l o c i t y components can be measured: U, the component i n the X d i r e c t i o n , and W, the component i n the Z d i r e c t i o n . The measurement of U or of W can be selected by r o t a t i n g the shaft by 90°. The standard deviation of v e l o c i t y i n the X and Z d i r e c t i o n s , u' and w' resp e c t i v e l y , are measured by the p.w.a., along with U and W. The coordinate system axes are defined as shown i n figure 5.3.3. X-0 at the plane of symmetry at the furnace center. Y-0 at the inside of the furnace front wall. Z-0 at the c e n t e r l i n e of the page 33 nozzles. FRDNTj V I E V t in ' ' ' ' ' ' q • looooaooaoP Plane Df Symmetry Steam Drum 10 f t 5 f t -0 f t --5 f t -SIDE V I E W ^ ^ M u o l Drun in i i gjDDOOQOoooooDaaaooaooig ° Limits Of g n Measurenent Area o -O.F.A. Nozzles - G r a t e Figure 5.3.3 Coordinate System and Region of Measurement with the P.W.A. page 34 5.4 Smoke Flow V i s u a l i z a t i o n For smoke flow v i s u a l i z a t i o n , two pieces of equipment were used: a smoke generator and a source f o r a l i g h t plane. The smoke generator produces a mist of f i n e o i l droplets that are sucked into the model by the vacuum ins i d e . The l i g h t plane was provided by a s l i d e p rojector. A s l i d e was covered with masking tape and a long, t h i n s l o t i s cut through i t . When the s l i d e was placed i n the projector, and the projector was po s i t i o n e d c o r r e c t l y near the model, a sheet of l i g h t varying i n thickness from about one h a l f of an inch at the w a l l near to the projector, to a maximum of about one and one h a l f inches at the opposite w a l l , was produced inside the model. page 35 6 Experimental Procedures Two sets of experiments were conducted with the apparatus. The v e l o c i t y f i e l d was traversed and measured with the pulsed wire anemometer (p.w.a.) and flow patterns were made v i s i b l e with smoke. During these experiments, furnace operating conditions were simulated by the appropriate s e t t i n g of the o v e r f i r e a i r (O.F.A.) and grate gas flow (G.G.F.) a i r flow rates i n the model. In s e c t i o n 6.1, the procedure f o r s e t t i n g O.F.A. and G.G.F. flow rates i n the model i s outlined. In sections 6.2 and 6.3, the procedures f o r p.w.a. measurement of the flow f i e l d and smoke flow v i s u a l i z a t i o n are outlined. 6.1 Model A i r f l o w Settings The following procedure i s used to determine the model O.F.A. and G.G.F. se t t i n g s : (O.F.A.) and (G.G.F.) are c a l c u l a t e d i n Appendix B p p Calculate f with equation 4.3.8. When choosing the magnitudes of (O.F.A.) and (G.G.F.) , the m in l i m i t a t i o n s of the blower and compressed a i r supply, the range of c a l i b r a t i o n of the o r i f i c e plates and the c a l i b r a t e d range of the p.w.a. probes must be considered. To set the flow rates: Determine the manometer readings that correspond to the required O.F.A. and G.G.F. flow rates, from the o r i f i c e p l ate c a l i b r a t i o n curves. page 36 Set the pressure regulator at about 20 l b / i n Adjust the O.F.A. flow co n t r o l valves u n t i l the required O.F.A. manometer readings are achieved. Turn • on the blower and adjust the v a r i a c u n t i l the required G.G.F. manometer reading i s obtained. Check the O.F.A. manometer readings and readjust i f necessary. 6.2 P.W.A. Procedures The procedures f o r measuring v e l o c i t y with the p.w.a. are described here. V e l o c i t i e s are measured at various locat i o n s i n several planes in s i d e the model. Each plane i s defined as Y=constant, or Z-constant and i s therefore v e r t i c a l or h o r i z o n t a l . The probe p o s i t i o n can be set i n 1/8 inch increments i n the Y and Z d i r e c t i o n s . The p o s i t i o n of the probe i n the X d i r e c t i o n depends on the spacing of the s l o t s i n the rear wall of the model. The p.w.a. c o n t r o l l e r u n i t i s operated by a menu driven program i n the computer. The program prompts f o r the time delay between samples, the l i m i t s of the c a l i b r a t e d range of the probe, the minimum acceptable time of f l i g h t and the l o c a t i o n of the v e l o c i t y measurement. The program p r i n t s out the measured mean v e l o c i t y and standard deviation of v e l o c i t y , the measurement l o c a t i o n , and numbers of rejected samples. The program signals the instrument to sample the v e l o c i t y repeatedly f o r each measurement. The program computes the mean page 37 v e l o c i t y , u, and the standard deviation of v e l o c i t y u'= \~ z from J u the v e l o c i t y samples. In order to obtain an accurate, repeatable v e l o c i t y measurement, four c r i t e r i a must be considered. There must be an adequate number of samples to get s t a t i s t i c a l l y s i g n i f i c a n t estimates of u and u' . The time over which the samples are taken must be long compared to the time scale of any o s c i l l a t i o n s i n the flow. The s i g n a l q u a l i t y must be good, and the v e l o c i t i e s must f a l l w i t h i n the c a l i b r a t e d range of the instrument. The procedures f o r obtaining accurate, repeatible measurements are d e t a i l e d i n Appendix A.4. A rough check of measurement of v e l o c i t i e s can be made by i n t e g r a t i n g the v e l o c i t i e s across the experimental cross s e c t i o n to c a l c u l a t e the flow rate. The r e s u l t can be compared to the actual flow rate used. In Appendix A.5, flow rate settings f o r the present model study are compared with flow rates c a l c u l a t e d by i n t e g r a t i n g the v e l o c i t i e s measured with the p.w.a. over the model cross section. 6.3 Smoke Flow V i s u a l i z a t i o n Flow patterns are simulated with smoke under conditions simulating various furnace loads. A hose connects the o u t l e t of the smoke generator to the o v e r f i r e a i r plenum on one side of the model. The plenum i s under a s l i g h t vacuum so that i t sucks smoke from the smoke generator. The amount of smoke can be c o n t r o l l e d with the smoke generator. The rate of gas flowing into the plenum from the smoke generator i s small compared the the O.F.A. flow rate. page 38 The plane i s illuminated with the s l i d e projector as explained i n se c t i o n 5.3. The model i s covered with black paper, and the room l i g h t s are turned o f f to minimize glare o f f of the p l a s t i c , and to reduce i l l u m i n a t i o n of areas outside of the l i g h t plane. Photographs are taken of the l i g h t plane with a 35mm si n g l e lens r e f l e x camera. page 39 7 Results The r e s u l t s of the pulsed wire anemometer experiments are presented as figures i n Appendix E. Selected smoke flow v i s u a l i z a t i o n photographs are i n Section 8. In t h i s section the v a r i a b l e s that are p l o t t e d i n the figures are defined and c a l c u l a t i o n s used to produce the figures are described. Variables of the smoke flow v i s u a l i z a t i o n are defined. In s e c t i o n 7.1, the model a i r f l o w settings are c a l c u l a t e d to simulate the furnace operating conditions, and the corresponding s c a l i n g f a c t o r s are c a l c u l a t e d to scale v e l o c i t i e s measured i n the model to the prototype. In s e c t i o n 7.2, p l o t s of the p.w.a. data and relevant c a l c u l a t i o n s are described. In s e c t i o n 7.3, photographs of smoke i n the model are described. 7.1 Model A i r Flow Rates Experiments were conducted at four settings of o v e r f i r e and undergrate a i r i n the model to simulate four operating conditions of the prototype furnace: case 1, 100% f u l l load; case 2, 71% f u l l load with dampers to the h o r i z o n t a l nozzles open; case 3, 71% f u l l load with dampers to the h o r i z o n t a l nozzles closed, and case 4, 60% f u l l load. The b o i l e r load i s the flow rate of steam generated i n the b o i l e r and d e l i v e r e d to the steam header. I t Is s p e c i f i e d i n pounds of steam per hour. F u l l load i s 150000 lb/hr. The r a t i o of the o v e r f i r e a i r flow rate to the t o t a l combustion a i r flow rate, expressed as a percent (%0.F.A.) ranges from 60% f o r case 1, page 40 to 31% f o r cases 2 and 3, to 0% f o r case 4. The o v e r f i r e a i r volume flow rate (O.F.A.), the flow rate of gases r i s i n g from the grate (G.G.F.) and other parameters i n the prototype furnace were c a l c u l a t e d f o r each operating condition i n Appendix B. The O.F.A. and G.G.F. settings f o r the model were s p e c i f i e d f o r maximum a i r v e l o c i t y , taking into account the range of c a l i b r a t i o n of the a i r flow instruments, and the l i m i t a t i o n s of the blower and compressed a i r supply, according to the s c a l i n g r u l e s of sec t i o n 4.3. The furnace operating conditions, O.F.A.p, G.G.F.p, O.F.A.m, v' G.G.F.m and ^f-, are i n table 7.1.1. The c a l c u l a t i o n of O.F.A.m, D V ' G.G.F.m and ^ 7 are shown f o r case 2 i n the following example, m The volume flow rate of a i r at the grate i n the model, G.G.F.m i s equal to the model under grate a i r volume flow rate, U.G.A.m, since no combustion occurs i n the model. G.G.F.m was chosen to be 51.5 f t / m i n . which i s near the maximum flow rate that could be provided by the blower. O.F.A.m and the v e l o c i t y s c a l i n g f a c t o r were c a l c u l a t e d as follows: ®MX- C ( cuat ion 4.3. 8) s p From Appendix B, ( — ) -2.173 E Also, from Appendix B, O.F.A.p-1683000 f t 3 / h r and G.G.F.p=11080000 f t 3 / h r . Therefore: ^ | " ) ^ =1683000/11080000x2.173-. 33012 page 41 O.F.A.m-. 33012x51.5 f t3/min.=17.00 f t 3 /min. The v e l o c i t y s c a l i n g f a c t o r i s : v' ^ " C G ^ F : . ) X S * (equation 4.3.11) m S=model scale=l/15 v ' 1 so that -11080173/(51.5x60)x Ti2=15. 93 Table 7.1.1 Flow Rates and Scalin g Factors i n the Prototype Furnace and the Model case 1 2 3 4 load 1000 lb/hr load 1000 slug/hr dampers to h o r i z o n t a l nozzles %0.F.A. o p e r a t i n g c o n d i t i o n 150 107 107 60 4.66 3.32 3.32 1.86 open open closed 60 31.4 31.4 0 f t 3 0.F.A.p 1000 =± hr f t 3 G.G.F.p 1000 i± hr p r o t o t y p e flow r a t e s 4496 1683 1683 0 10857 11080 11080 8254 f t 3 O.F.A.m i f min G.G.F.m ^ min model flow r a t e s 45.0 17.0 17.0 0 50 51.5 51.5 51.5 v' p v' m s c a l i n g f a c t o r 16.08 15.93 15.93 11.87 7.2 Plots of the P.W.A.Results V e l o c i t i e s were measured f o r cases 1, 2, 3 and 4. In each case, the measured v e l o c i t i e s and turbulence i n t e n s i t i e s are page 42 V m u l t i p l i e d by —y-. The X, Y and Z p o s i t i o n s of the traverse m locatio n s where v e l o c i t i e s were measured were m u l t i p l i e d by 1/S=15. The X Y and Z coordinate axes i n a l l p l o t s i n Appendix E are defined as shown i n fig u r e 5.3.3. V e l o c i t i e s were p l o t t e d as vectors f o r a l l four cases i n Appendix E . l . In these figures, the posit i o n s of the o v e r f i r e a i r nozzles are indi c a t e d by open c i r c l e s , and the p o s i t i o n s of the nozzles angled downwards 15° from h o r i z o n t a l are i n d i c a t e d by s o l i d black c i r c l e s . The nozzles of the model and the prototype are discussed i n section 5 and Appendix B. V e l o c i t y p r o f i l e s p l o t t e d with Y as the abscissa are i n Appendix E.2. V e l o c i t y p r o f i l e s with X as the abscissa are i n Appendix E.3. Plots of average turbulence i n t e n s i t y , v', as well as u' i n various planes are i n Appendix E.4., where: v»-H^2' 7.1.1 Plots of the isotropy r a t i o , R are i n Appendix E.5 where: R=H; 7.1.2 w' J e t t r a j e c t o r i e s and ce n t e r l i n e speeds i n various v e r t i c a l planes defined by Y=constant, are p l o t t e d on the figures i n Appendix E.6. V e l o c i t i e s were measured at f i v e values of X: X=-.25, 1.625, 3.5 5.375 and 7.25 feet i n prototype u n i t s , where X=0 at the plane of symmetry. The furnace side wall on which the nozzles are mounted i s located at X-8.5 f t . In the figures of Appendix E.6, X was transformed as X'-8.5-X so that the abscissa equals 0 at the nozzles, and 8.5 at the plane of symmetry. Speeds page 43 and v e r t i c a l displacements measured at X=-.25 fee t are r e f l e c t e d back to X=.25 f t . , or X'=8.25 f t . At each traverse l o c a t i o n i n the plane Y-constant, the gas 1/2 speed s=f (X',Z)=(U2+W2) was calcul a t e d . For each X traverse p o s i t i o n , the maximum speed s=f(X') was found, s (X') i s p l o t t e d m m i n the fig u r e s i n Appendix E.6 as a bar graph, t i t l e d 'EXPERIMENT'. The locus of points Z(X') where s-s i s p l o t t e d as a ID curve, t i t l e d 'LOCUS OF MAXIMUM SPEED'. The v e l o c i t y vectors are p l o t t e d i n the figures as we l l . Also p l o t t e d i n the figures i n Appendix E.6 are values f o r Z(X') and sm(X'). Z(X') i s predicted with equation C.5.1.18, which models a si n g l e round j e t i n unconfined, uniform crossflow. This i s t i t l e d 'SINGLE ROUND NOZZLE': 7 1 15 Y 2 , 6 * | o-(M) [ U (from equation C.5.1.18) 2 p V M=» — — P 2 0 v , the average v e r t i c a l v e l o c i t y of gas r i s i n g from the 8 G G F 2 grate i s estimated by * A* , where Ag=297.5 f t i s the ho r i z o n t a l cross s e c t i o n a l area of the furnace above the grate. v o 0 F A i s the v e l o c i t y at the nozzle. I t i s estimated as v - * ' ' , J 0 AO where Ao i s the t o t a l cross s e c t i o n a l area of the o v e r f i r e a i r nozzles to which the dampers are open, do, the nozzle diameter i s estimated by the average diameter of the nozzles to which the dampers are open. Ao, M, v , v and do are tabulated i n Appendix p 8 E.7 f o r cases 1, 2 and 3. -^-2.173 i s the r a t i o of the o v e r f i r e P page 44 a i r density divided by the density of the gas r i s i n g from the grate. sm(X') i s predicted with equation C.5.1.13 and t i t l e d 'ROUND JET', i n the figures of Appendix E.6. I t i s c a l c u l a t e d as follows: p 1/2 , — - 6.3 f— ) •„ g J .X > 8do (from equation C.5.13) v ^p J (X + . 6do) v -l o g — - 1 X < 8do v 0 which i s v a l i d f o r a si n g l e round j e t i n unconfined, quiescent flow. sm(X') i s also predicted with equation C.5.1.18 and t i t l e d 'SLOT JET' i n the figures of Appendix E.6. I t i s c a l c u l a t e d as follows: -1/2 p 1/2 Sm -2.48 ( - + .6 ] [ — ] X > 8do (from equation Vo yo p g C.5.1.18) — - 1 X < 8do v 0 which i s v a l i d f o r a j e t i s s u i n g from a s l o t of width y Q, and i n f i n i t e length, i n unconfined quiescent flow. In the fi g u r e s , y Q Ao i s estimated as y - — . L equals the h o r i z o n t a l distance between Jo L ^ the centers of the front and rear O.F.A. nozzles. L-10.69 f t . y o i s tabulated i n Appendix E.7. sm and Z are c a l c u l a t e d i n Appendix E.7 as an example. 7.3 Smoke Flow V i s u a l i z a t i o n Results Photographs were taken of smoke i n the flow f i e l d with the model a i r flow rates set as i n cases 1, 2 and 3, described i n sec t i o n 7.1. Smoke was introduced as described i n s e c t i o n 6.3. Selected smoke flow v i s u a l i z a t i o n photographs are i n fig u r e s 8.2.2 page 45 to 8.2.4. Smoke was photographed i n v e r t i c a l planes corresponding approximately to Y=8 f t . , and Y=15 f t . i n prototype furnace u n i t s . The h o r i z o n t a l l i n e s on the rear wall of the model correspond to 2 f t . increments i n the prototype furnace. The v e r t i c a l l i n e s correspond to X-0, 1.625, 3.5, 5.375 and 7.25 f t . from l e f t to r i g h t . Photographs were taken of smoke i n v e r t i c a l planes corresponding approximately to X-5.4 f t . The top of the bullnose i s at the top l e f t of these photographs at Z-21 f t . When case 1 was simulated, i t was observed that the row of o v e r f i r e a i r j e t s on each side of the plane of symmetry o s c i l l a t e d with respect to each other. The banks of j e t s would alternate from the l e f t j e t s being above the r i g h t j e t s , at the plane of symmetry, to the j e t s on both sides c o l l i d i n g head on at the plane of symmetry, to the r i g h t j e t s being above the l e f t j e t s at the plane of symmetry. The displacement of the j e t s from the mean flow p o s i t i o n of c o l l i d i n g head on at the plane of symmetry however, was small. The period of the o s c i l l a t i o n was not measured, but i t was estimated to be i n the order of one second by v i s u a l observation. The period of o s c i l l a t i o n i n the prototype furnace can be estimated as follows: ( L ) " ( L) T J m *" T V 0 0 T= the period of o s c i l l a t i o n r -o v S page 46 £g— furnace length scale v = mean v e l o c i t y of gas r i s i n g from the grate s therefore, I z - ^ - ^ x ^ - i x ^ r r t g m v S v' m On gp n> V ' —7 - v e l o c i t y s c a l i n g f a c t o r m V ' For case 1: S-l/15, —7 -16.08 and r « 1 second. Therefore, V m m T — 1/15x16.08x 1 second «* 1 second as well, p page 47 8 Discussion of Results In t h i s section, the r e s u l t s are discussed with emphasis on material presented i n section 3 and Appendix C. In s e c t i o n 8.1, the gas flow f i e l d near the grate i s discussed, along with the e f f e c t s on combustion and the d i s t r i b u t i o n of f u e l on the grate. In s e c t i o n 8.2, mixing i n the flow f i e l d and gaseous combustion are discussed. Aerodynamics, in c l u d i n g j e t models, r e c i r c u l a t i o n , and channeling are discussed i n s e c t i o n 8.3. Buoyancy e f f e c t s are discussed i n s e c t i o n 8.4. While complete sets of figures are i n Appendices E . l to E.7 as already noted, key figures f o r the present discussion are reproduced i n the text and given consecutive numbers. The reader i s r e f e r r e d to the appendices i f more d e t a i l i s required. References to the material i n the appendices may be ignored however, without a loss of c o n t i n u i t y of the discussion. 8.1 Gas Flow Above the Grate Gas v e l o c i t i e s were measured i n the plane Z--4.5 f t . f o r cases 1 and 4 (150k lb/hr., 60% O.F.A., and 60k lb/hr., 0% O.F.A. r e s p e c t i v e l y ) . The plane Z--4.459 f t . i s 2.4 fe e t above the grate at the rear of the furnace, and 4 feet above the grate at the fr o n t of the furnace. I t i s the lowest plane i n which v e l o c i t i e s were measured. Important differences between cases 1 and 4 are that case 1 has o v e r f i r e a i r and case 4 does not, and G.G.F.p i s higher i n case 1 than i t i s i n case 4. From table 7.1.1: G. G. F. P l c l 0 8 5 7 X l o 3 / 8 2 5 4 X i o V i . . 31 G.G.F.p* where the subscript 1 r e f e r s to case 1, and the page 48 subscript 4 r e f e r s to case 4. I f the G.G.F.p^ was increased to the G.G.F.pi, with no o v e r f i r e a i r , the flow f i e l d would remain homologous to case 4, since the flow i s turbulent, and Reynold's number e f f e c t s are n e g l i g i b l e . Therefore v ' -1.31 v ' , i f there i s no o v e r f i r e a i r . pi p 4 Thus, the magnitude of the v e l o c i t i e s of case 1 are increased above the v e l o c i t i e s of case 4 by a f a c t o r of 1.31, and any other changes between case 1 and case 4 can be a t t r i b u t e d to the o v e r f i r e a i r j e t s . I t i s therefore worthwhile to examine case 4 f i r s t , because i t i s simpler, and then to see from the r e s u l t s f o r case 1, what the added e f f e c t s of increased gas flow, and o v e r f i r e a i r are on the flow. Two v e l o c i t y p r o f i l e s i n the plane Z—4.459 f t . , at X-1.6ft. and X-5.4 are p l o t t e d i n Appendix E.7 f o r case 4. The p r o f i l e at X-1.6 f t . i s presented i n f i g u r e 8.1.1. page 49 Y , F T . Figure 8.1.1 V e l o c i t y P r o f i l e , Case 4, X-1.6 f t , Z—4.5 f t Since there i s no o v e r f i r e a i r , t h e h o r i z o n t a l mean v e l o c i t y component, U i s usu a l l y much less than the v e r t i c a l mean v e l o c i t y component, W. The p r o f i l e i n figu r e 8.1.1 can be divided into 4 regions. Region 1, from Y=3 f t . to Y-=6 f t . i s a region of peak v e r t i c a l v e l o c i t y . The peak v e l o c i t y i s about 55 f t . / s . At t h i s value of W, T (the thic k e s t p a r t i c l e that would be entrained, Appendix C.2) was c a l c u l a t e d with equation 4.2.1 assuming wet douglas f u r as the f u e l , and found to equal .25 inch. Thus, f u e l p a r t i c l e s l e s s than .25 inches t h i c k w i l l not s e t t l e at the point of peak v e l o c i t y , and smaller p a r t i c l e s w i l l not tend to s e t t l e i n region 1 at a l l . Region 2, from Y-6 f t . to Y=15 f t . i s a region of low v e r t i c a l v e l o c i t y . In region 2, W i s less than 25 f t . / s . The under grate a i r enters the furnace through a 4 f t . wide by 3 f t . high, rectangular opening i n the rear of the under grate page 50 a i r plenum. The a i r that flows through the opening forms a confined j e t under the grate that spreads out towards the sides of the plenum as i t reaches the front of the plenum. At the front of the plenum, the j e t i s d e f l e c t e d upwards, through the grate, r e s u l t i n g i n a region of high W near the f r o n t of the grate. A i r i s entrained into the j e t near the rear of the grate, r e s u l t i n g i n low or negative W there. Thus, W i s high i n region 1, and low or negative i n region 2. In the prototype furnace, there i s a distance of 1.25 f t . between the f r o n t wall, and the f i r s t row of holes i n the grate. There i s also a distance of .8 f t . between the l a s t row of holes i n the grate, and the rear w a l l . As a r e s u l t , model measurements reveal a r e c i r c u l a t i o n zone i n region 3, between Y-0 f t . and Y-4 f t . In the r e c i r c u l a t i o n zone, W i s negative near the f r o n t wall and increases to the peak v e l o c i t y of about 55 f t . / s . near Y-6 f t . A s i m i l a r r e c i r c u l a t i o n zone may e x i s t i n region 4, between Y=16 f t . and the rear w a l l , where Y-17.5 f t . , but i t i s not obvious from f i g u r e 8.1.1. The s i z e of the r e c i r c u l a t i o n zone of region 3 i n the model may be somewhat exaggerated because the spacing between the grate holes i s much larger than the homologous spacing i n the prototype furnace. Consequently the f i r s t row of grate holes i s a c t u a l l y 2 ft.from the front wall i n the model, when scaled up to the prototype s i z e . Nevertheless, the model r e s u l t s are i n d i c a t i v e of the prototype flow pattern. I t can be concluded that, with no o v e r f i r e a i r , the v e l o c i t y p r o f i l e s near the grate are non-uniform; they are dominated by the page 51 j e t underneath the grate, and the r e c i r c u l a t i o n zones caused by areas of the grate, near the walls, i n which there are no holes. Fuel p a r t i c l e s w i l l tend to p i l e i n areas of low or negative v e r t i c a l v e l o c i t y leaving the areas on the grate of high W, around Y=3 f t . to 6 f t . bare. As described i n Appendix C.2, uneven f u e l p i l i n g on the grate would cause the peak v e l o c i t i e s to become even higher, r e s u l t i n g i n even worse d i s t r i b u t i o n of the f u e l p i l e . Also, as described i n Appendix C.2, uneven f u e l p i l e d i s t r i b u t i o n leads to uneven d i s t r i b u t i o n of combustion a i r to f u e l on the grate, r e s u l t i n g i n increased requirements of undergrate a i r to s u s t a i n stable combustion of wet f u e l , and therefore, increased p a r t i c l e entrainment and carryover. V e l o c i t y p r o f i l e s f o r case 1 are p l o t t e d i n Appendices E.2.1 to E.2.5 f o r various values of X. The bottom graph i n each figure corresponds to Z--4.459 f t . The shapes of the curves f o r Z--4.459 f t . , case 1 are s i m i l a r to that of case 4, f i g u r e 8.1.1. They can be divided into the same 4 regions as before. The peak v e l o c i t i e s f o r case 1 at X-1.625 f t . and 5.375 f t . are about 80 f t . / s . and 90 f t . / s . r e s p e c t i v e l y . However, i f the peak v e l o c i t y of case 4 i s v ' m u l t i p l i e d by — ^ 7 - , the predicted peak v e l o c i t y i s only 1.31x55=72 f t . / s . At Z--4.459 f t . U i s much higher f o r case 1 than i t i s for case 4. Also, f o r case 1, at X-1.625 f t . , W i s negative over most of region 2, with a peak negative magnitude of about 55 f t . / s . W from case 4, at X-1.625 f t . , i s small and p o s i t i v e over most of region 2. The o v e r f i r e a i r j e t s , therefore, have a s u b s t a n t i a l page 52 e f f e c t on the gas flow over the grate. In f i g u r e 8.1.2, W i s p l o t t e d as a function of X at Z—4.46 f t . , Y»8.1 f t . and r e s u l t s are p l o t t e d f o r cases 1, 2, and 4. Overf i r e a i r j e t s have the e f f e c t of causing W to be large near the furnace walls, and small near the plane of symmetry. The e f f e c t appears to increase as the flow rate of o v e r f i r e a i r i s increased. X, FT. Figure 8.1.2 P r o f i l e of V e r t i c a l V e l o c i t y W, Cases 1, 2 and 4, Z—4.5 f t , Y=8.1 f t The o v e r f i r e a i r nozzles are positioned on the side walls of the furnace as shown i n figu r e B . l . l . There i s a distance of 5.6 f t . between the nozzle nearest the front of the furnace, and the fr o n t w a l l . The space was included to prevent f u e l from becoming entrained i n the o v e r f i r e a i r j e t s as i t f e l l from the hoppers. V e l o c i t y vectors i n v e r t i c a l planes are p l o t t e d i n Appendices E . l . l to E.1.3 f o r case 1. The vectors i n the planes Y-8.1 f t . and Y=2.5 f t . are p l o t t e d i n figures 8.1.3 and 8.1.4 r e s p e c t i v e l y . The bottom most vectors i n these p l o t s are i n the plane Z--4.46 f t . Figure 8.1 Y-8.1 f t .3 Case 1, V e l o c i t y i n the Plane page 55 From fig u r e 8.1.3, I t can be seen that the plane Z=-4.459 f t . i s i n the r e c i r c u l a t i o n zone of the o v e r f i r e a i r j e t s . The d i r e c t i o n of the v e l o c i t y near the plane of symmetry (X=0 f t . ) i s downwards. As X increases, the v e l o c i t y becomes h o r i z o n t a l at about X=3.5 f t . , and near the wall (X-8.5 f t . ) , the v e l o c i t y i s upwards. Figure 8.1.4, however, shows no r e c i r c u l a t i o n zones, or j e t s . The v e l o c i t y i s almost v e r t i c a l everywhere i n the plane shown. The plane Y=2.5 f t . , which i s p l o t t e d i n fi g u r e 8.1.4, i s between the fr o n t w a l l and the fr o n t o v e r f i r e a i r nozzles. I t i s therefore apparent that the space between the front o v e r f i r e a i r nozzle, and the front w a l l acts to change the p r o f i l e s of W along the d i r e c t i o n of the Y ax i s . Near the plane of symmetry, a i r moving downwards towards the grate from the r e c i r c u l a t i o n zones of the o v e r f i r e a i r j e t s drives gas upwards near the fro n t , where there are no o v e r f i r e a i r j e t s . Increased peak values of W, and increased negative v e r t i c a l v e l o c i t i e s near the rear of the furnace are the r e s u l t s apparent from the fi g u r e s . The flow above the grate i s i l l u s t r a t e d schematically i n figu r e 8.1.5. page 56 .RECIRCULATION ZDNES GRATE ENTRANCE FDR -UNDERGRATE AIR Y CONFINED JET UNDER GRATE Z t_*,X a.,, _JNDVERFIRE AIR RECIRCULATION ZONES NOZZLES OF OVHRFIIRB: t TV AIR JE/*S V J 1 x G R A T E I s c . / I ^~rY\\/ M LT_t_._ 1 J J _ * J ENTRANCE FOR UNDERGRATE AIR [ ^ V E R T I C A L VELOCITY ABOVE THE GRATE Figure 8.1.5 Flow Pattern Above the Grate page 57 The following conclusions can be drawn: (i) The confined j e t under the grate, formed when the under grate a i r enters the plenum through a small opening r e s u l t s i n non uniform v e l o c i t y p r o f i l e s above the grate. When coupled with the e f f e c t of r e c i r c u l a t i o n zones caused by areas of the grate near the furnace walls with no holes, peak v e r t i c a l v e l o c i t i e s of 55 ft./s. are produced about 6 f t . from the f r o n t w a l l . Near the f r o n t w a l l , the d i r e c t i o n of flow i s downwards, and a region i n which the v e r t i c a l v e l o c i t y component i s low or negative occurs from 8 f e e t from the front w a l l to the rear w a l l . (ii> The r e c i r c u l a t i o n zones of the o v e r f i r e a i r j e t s cause v e l o c i t y to be upwards near the walls, down near the plane of symmetry, and h o r i z o n t a l i n between. ( i i i ) The 5.6 foot space between the f r o n t o v e r f i r e a i r nozzle and the f r o n t wall causes peak upwards v e l o c i t i e s near the grate to increase, and areas of downwards v e l o c i t y to occur above the rear h a l f of the grate. (iv) Uneven v e l o c i t y p r o f i l e s cause uneven f u e l p i l i n g which i n turn makes v e l o c i t y p r o f i l e s more uneven. Non-uniform d i s t r i b u t i o n of a i r to the f u e l p i l e increases the under grate a i r requirement f o r the stable combustion of wet f u e l on the grate, r e s u l t i n g i n increased entrainment and carryover. I t i s recommended that, i n hog f u e l b o i l e r s : (i) O v e r f i r e a i r nozzles should be spaced along the furnace side walls, with no large space between the front o v e r f i r e a i r nozzles and the f r o n t w a l l . page 58 (ii) Areas of the grate near the wall, with no holes should be as small as p o s s i b l e . ( i i i ) The grate and undergrate plenum should be c a r e f u l l y designed to produce uniform v e l o c i t y p r o f i l e s above the grate. This could be done with a scale model study. 8.2 Mixing In t h i s section, macro scale mixing of gases i s addressed with the smoke flow v i s u a l i z a t i o n r e s u l t s and with v e l o c i t y p r o f i l e s near the furnace e x i t . Micro scale mixing i s addressed with p l o t s of turbulence i n t e n s i t y . Macro scale mixing i s the production of uniform time averaged p r o f i l e s of gas concentration and other v a r i a b l e s , across the furnace section. Micro scale mixing i s the reduction of deviations of instantaneous gas concentrations from the time averaged values. Macro and micro mixing are discussed i n Appendix C.3. Smoke Flow V i s u a l i z a t i o n The process of mixing smoke and a i r i n the model i s i l l u s t r a t e d i n f i g u r e 8.2.1. Smoke f i l l e d a i r enters through the bank of nozzles at the r i g h t . A smoke f i l l e d j e t occurs i n region 1. Clear a i r enters through the bank of nozzles on the l e f t forming a j e t i n region 2. Clear a i r also enters through the grate at the bottom. Clear a i r , and smokey a i r mix i n regions 3 and 4. Some smokey a i r from regions 3 and 4 i s entrained into the j e t of region 2. page 59 — Point ~T _ 1 Over f i re Air Nozzles Smoke Fuel Gra te Figure 8.2.1 Smoke Flow V i s u a l i z a t i o n Regions i n the Model Smokey a i r i s grey i n the photographs, and regions of c l e a r a i r appear to be black. When the r a t i o of O.F.A. divided by G.G.F. i s large, region 2 appears to be black, and when the r a t i o i s low, region 2 appears to be grey when compared to regions 3 and 4. The effectiveness of macro scale mixing can be judged q u a l i t a t i v e l y by the uniformity of greyness i n regions 3 and 4, and q u a n t i t a t i v e l y by the height of point P (see f i g u r e 8.2.1), at the i n t e r s e c t i o n of regions 1, 2 and 3. The height of point P in d i c a t e s how high above the o v e r f i r e a i r nozzles gases from the grate and o v e r f i r e a i r must r i s e before they s t a r t to mix. Photographs of smoke s i m i l a r to f i g u r e 8.2.1 were taken for cases 1, 2 and 3. There are two basic types of flow patterns, t y p i f i e d by f i g u r e s 8.2.2 and 8.2.3. Air page 60 Figure 8.2.2 Case 2, Smoke i n the Plane Y-8 f t Figure 8.2.2 i s of smoke i n the plane Y-8 f t . , f o r case 2 (107k lb/hr, 31% o v e r f i r e a i r , dampers open to a l l nozzles). The plane i s illuminated from above, so that a trapezoidal area, bounded by the top of the photograph, the grate and two rays that s t a r t at the l i g h t source, i s illuminated. Areas of the photographs l i e outside the trapezoid and are therefore not illuminated, as can be seen from the f i g u r e . The height of P above the o v e r f i r e a i r nozzle cen t e r l i n e s , Zp can be estimated i n prototype furnace Z coordinate units from the g r i d at the back of page 61 the model, described i n section 7.2. Estimates of Zp are approximate because of para l l a x error, and because the boundaries between regions 1, 2, and 3 are not c l e a r l y defined. Figure 8.2.3 Case 1, Smoke i n the Plane Y-15 f t Zp i s about 8 to 10 feet i n figure 8.2.2 Gases from the grate and o v e r f i r e a i r can not mix thoroughly u n t i l they r i s e at l e a s t 8 feet above the o v e r f i r e a i r nozzles. Region 3 appears to be smoke free so that l i t t l e mixing occurs below the nozzles. Region 4 appears to be l i g h t e r on the r i g h t side than on the l e f t , even near the top of the bullnose at the tops of the photographs, page 62 i n d i c a t i n g that gases do not mix thoroughly before e x i t i n g the furnace. A s i m i l a r flow pattern was observed f o r case 2 at Y=15 f t . Figure 8.2.3 i s of the plane Y-15 f t . , case 1 (150k lb/hr, 60%O.F.A.). Zp=0 f t . The j e t of region 2 i s very dark when compared to i t s surroundings, and i t appears to c o l l i d e with the j e t of region 1 at the l e v e l of the nozzles. Thus, gases have more time to mix before e x i t i n g the furnace than i n case 2. Region 3 i s grey, i n d i c a t i n g that gases above the grate mix with o v e r f i r e a i r . Region 4 appears to be a uniform shade of grey, i n d i c a t i n g that the mixture of gases near the furnace e x i t i s more uniform than i n case 2. A s i m i l a r flow pattern was observed i n case 1, Y-8 f t . Photographs of smoke i n the planes Y-8 f t . and Y= 15 f t . , f o r case 3 (105k lb/hr, 31% O.F.A., dampers to h o r i z o n t a l nozzles closed) were taken but not Included here. At Y—8 f t . , Zp equals 8 to 10 feet , and mixing appears to be much l i k e the open damper case 2 shown i n fi g u r e 8.2.2. However, at Y-15 f t . , a flow pattern very s i m i l a r to fi g u r e 8.2.3 occurs. Zp-0 f t . and the j e t s of region 1 and region 2 appear to c o l l i d e at the l e v e l of the o v e r f i r e a i r nozzles. However, the shade of region 4 appears to be somewhat uneven, i n d i c a t i n g l e s s complete mixture of gases there than occurs i n case 1. The d i f f e r e n c e between the pattern of mixing i n the two planes, Y=8ft. and Y -15 f t . , f o r case 3, can be a t t r i b u t e d to the e f f e c t of the space between the front o v e r f i r e a i r nozzles and the furnace f r o n t w a l l , since the plane of Y-8 f t . i s close to the front end of the bank of o v e r f i r e a i r j e t s and the plane Y-15 f t . page 63 i s not. The e f f e c t of the space between the front o v e r f i r e a i r nozzles, and the front wall i s i l l u s t r a t e d by f i g u r e 8.2.4. The photograph i s of the plane X=5.4 f t . The top of the bullnose i s at the top l e f t corner of the photo, the furnace rear wall i s at the l e f t side of the photo, and the furnace front wall i s on the r i g h t side. The bottom of the photo i s at about Z=8 f t . i n prototype furnace u n i t s . The plane i s illuminated from the r i g h t . The operating condition of the furnace i s case 2. page 64 Figure 8.2.4 Case 2, Smoke i n the Plane X-5.4 f t On the r i g h t , near the front wall i s a dark region, while the re s t of the plane i s grey with smoke. This indicates that, since there are no o v e r f i r e a i r j e t s near the front wall of the furnace, gases from the grate that r i s e near the front wall do not mix with the o v e r f i r e a i r , or the gases r i s i n g above the o v e r f i r e a i r j e t s i n region 3. In the prototype furnace, the grey area i s r i c h e r i n oxygen than the dark area. Emissions from the furnace would be reduced i f the oxygen concentration was more uniform. A s i m i l a r page 65 observation was made for case 1. The same e f f e c t appears to occur, but to a l e s s e r extent. From the smoke photographs, i t can be concluded that there i s a v e r t i c a l s t r a t i f i c a t i o n of the flow i n the furnace; near the front w a l l gases are lower i n oxygen than they are away from the fro n t w a l l , above the o v e r f i r e a i r nozzles. The s t r a t i f i c a t i o n occurs because of the 5.6 foot space between the fr o n t w a l l and the f r o n t o v e r f i r e a i r nozzles. Gases r i s i n g from the grate near the f r o n t w a l l do not mix with o v e r f i r e a i r , or the gases above the o v e r f i r e a i r j e t s i n region 3, because they do not pass through the bank of o v e r f i r e a i r j e t s . The s t r a t i f i c a t i o n i s probably worsened by the uneven v e l o c i t y d i s t r i b u t i o n above the grate discussed i n section 8.1. Eli m i n a t i o n of the s t r a t i f i c a t i o n would r e s u l t i n a lower requirement f o r combustion a i r , and therefore, reduced p a r t i c l e entrainment and carryover, and higher e f f i c i e n c y . The s t r a t i f i c a t i o n could be reduced by the placement of o v e r f i r e a i r nozzles near the front wall, and by improving the design of the under grate plenum to produce a uniform v e l o c i t y d i s t r i b u t i o n as described i n sec t i o n 8.1. Mixing of gases above and underneath the o v e r f i r e a i r j e t s i s good i n case 1 (60% o v e r f i r e a i r and dampers open to a l l nozzles) and case 3 (31% o v e r f i r e a i r and dampers open to one h a l f of the nozzles on each side of the furnace only) except near the front w a l l . Mixing i s poorer i n case 2 (31% o v e r f i r e a i r , dampers open to a l l nozzles). The c l o s i n g of dampers to some nozzles at low page 66 o v e r f i r e a i r flow rates, to maintain high v e l o c i t y at the nozzles that are open, so that Zp i s close to 0 f t . i s recommended. V e l o c i t y P r o f i l e s I f the v e l o c i t y p r o f i l e i s not uniform across the furnace i n the combustion zone, gas and p a r t i c l e s i n the regions of high v e r t i c a l v e l o c i t y may be quickly ejected from the furnace before combustion i s complete. Intense mixing however, w i l l t r a n s f e r gas and p a r t i c l e s out of the high v e r t i c a l v e l o c i t y regions once entrained, lengthening residence time. Intense mixing i n the combustion zone should r e s u l t i n r e l a t i v e l y uniform v e l o c i t y p r o f i l e s near the furnace e x i t . Mixing i s provided by o v e r f i r e a i r j e t s . I t i s therefore worthwhile to examine v e l o c i t y p r o f i l e s near the furnace e x i t and i n the combustion zone to determine the effectiveness of mixing of momentum i n the furnace. W i s p l o t t e d as a function of X at Z-13.3 f t , Y-8.1 f t . f o r case 1 (150k lb/hr, 60% o v e r f i r e a i r ) and case 2 (107k lb/hr, 31% o v e r f i r e a i r ) i n figu r e 8.2.5. In case 1, W decreases from about 16 f t . / s . at the ce n t e r l i n e , to near 0 f t . / s . at X-7.25 f t . , i n d i c a t i n g that the plane Z-13.3 f t . i s influenced by the r e c i r c u l a t i o n zone above the o v e r f i r e a i r j e t s . The top row of vectors i n fi g u r e 8.1.3 i s i n the plane Z—13.3 f t . , so that the r e l a t i o n s h i p between the plane and the r e c i r c u l a t i o n zone can be seen. page 67 X, FT Legend A CASt ICOK LB/Xfi., 6QgQ.rj>.  X CAST 2 K>7K LB/HR., 3t% O.F.A., PAMPERS OPCW Figure 8.2.5 P r o f i l e of W, Cases 1 and 2, Z=13.3 f t , Y-8.1 f t For case 2, from f i g u r e 8.2.5, at Z=13.3 f t . , i t can be seen that the p r o f i l e of W i s very non uniform. W ranges from 5 to 38 f t . / s . The v e l o c i t y p r o f i l e , therefore i s much more uniform i n the d i r e c t i o n of the X axis, f o r case 1, than i t i s f o r case 2. I t i s apparent that the v e l o c i t y p r o f i l e , near the furnace e x i t becomes more uniform along the d i r e c t i o n of the X axis, as the o v e r f i r e a i r flow rate i s increased. V e l o c i t y p r o f i l e s along the d i r e c t i o n of the Y axis f o r cases 1, 2 and 4 (60k lb/hr, no o v e r f i r e a i r ) are i n Appendix E.2. Measurements were taken at Z=1.625 f t . , which i s i n the combustion zone. The v e l o c i t y p r o f i l e f o r case 4 at Z=1.625 f t . , X-7.25 f t . i s p l o t t e d i n f i g u r e 8.2.6. The p r o f i l e f o r case 4, at X=3.5 f t . i n Appendix E.7 i s very s i m i l a r . page 68 I/) O O _ J LxJ > < 60--20-• \ N Legend A u x w ^ i A • >/v_ _ — -53 £s— — n — ' J LA 8 10 12 14 Y , F T . 16 18 Figure 8.2.6 V e l o c i t y P r o f i l e , Case 4, Z-1.6 f t , X-7.3 f t The e f f e c t of o v e r f i r e a i r j e t s on the flow f i e l d can be examined by comparing the r e s u l t s of case 1, with the r e s u l t s of v ' case 4, scaled by as was done i n sec t i o n 8.1 f o r flow above p« the grate. The e f f e c t of o v e r f i r e a i r j e t s on the v e l o c i t y p r o f i l e at Z-1.625 f t . can be demonstrated with a comparison of v e l o c i t y p r o f i l e s at Z-1.625 f t , case 1 ( i n Appendix E.2) with fi g u r e 8.2.6. At X=7.3 f t . , which i s near the furnace w a l l , there i s a q u a l i t a t i v e , as well as a quantitative d i f f e r e n c e i n the p r o f i l e s of W. Case 1 has a region of negative v e r t i c a l v e l o c i t y from Y-5.5 f t . to Y-16.5 f t . The peak values of W f o r case 1 and case 4 at X-7.3 f t . are 70 f t . / s . and 45 f t . / s . r e s p e c t i v e l y . I f the peak v ' W of case 4 i s m u l t i p l i e d by —^7-, the r e s u l t i s 1.31x45=59 f t . / s . Thus, o v e r f i r e a i r j e t s a l t e r the v e l o c i t y p r o f i l e r a d i c a l l y and page 69 increase the peak v e l o c i t y . Near the nozzles, the o v e r f i r e a i r j e t s tend to channel gas into the region near the fr o n t wall where there are no o v e r f i r e a i r nozzles, enhancing the non uniformity of the flow caused by the non uniform v e l o c i t y p r o f i l e above the grate that e x i s t s with no o v e r f i r e a i r . At X=3.5 f t . , Z=l .6 f t . , the o v e r f i r e a i r j ets have the opposite e f f e c t . From Appendices E.2.3 and E.2.7, i t can be seen that the p r o f i l e of W i s a c t u a l l y made more uniform, as the o v e r f i r e a i r j e t s i n t e r a c t with the region near the fr o n t wall where there are no o v e r f i r e a i r j e t s . Above the o v e r f i r e a i r nozzles, therefore, a region of high v e r t i c a l v e l o c i t y occurs near the front w a l l . The region of high v e l o c i t y i s caused by the confined j e t under the grate, and zones of the grate with no holes, as discussed i n se c t i o n 8.1. I t i s enhanced near the side walls and attenuated near the plane of symmetry, by the i n t e r a c t i o n of the o v e r f i r e a i r j e t s and the region near the fr o n t wall i n which there are no o v e r f i r e a i r j e t s . P r o f i l e s along the d i r e c t i o n of the Y axis f o r case 1, Z - l l f t . , which i s the highest plane i n which v e l o c i t y p r o f i l e s i n the d i r e c t i o n of the Y axis were measured, are p l o t t e d i n Appendices E.2.1 to E.2.5. At Z - l l f t . the p r o f i l e s of W are not uniform; W i s high near the fr o n t wall of the furnace and low elsewhere. W ranges from 0 to 55 f t . / s . I t can be concluded therefore, that a region of high v e r t i c a l v e l o c i t y occurs near the front wall of the furnace, even well page 70 above the o v e r f i r e a i r nozzles, caused by the 5.6 foot space between the f r o n t wall and the center l i n e of the f r o n t o v e r f i r e a i r nozzles, and the non uniform v e l o c i t y p r o f i l e above the grate. Strong r e c i r c u l a t i o n zones occur at high o v e r f i r e a i r flow rates producing a non uniform v e l o c i t y p r o f i l e . Intense mixing occurs i n t h i s case. Ove r f i r e a i r j e t s of low v e l o c i t y , l i k e those of case 3, r e s u l t i n a very non uniform v e l o c i t y d i s t r i b u t i o n i n the X d i r e c t i o n , even near the furnace e x i t , with l i t t l e mixing. Turbulence Intensity and Micro Scale Mixing Gradients i n the shear layers of the o v e r f i r e a i r j e t s produce turbulence. Small scale turbulence mixes gases intimately, on a micro scale. The rate at which turbulence i s produced i n the shear layers of the j e t s depends on the flow energy of the a i r at the e x i t of 2 v the o v e r f i r e a i r nozzles. The flow energy -moX_o where mo=the 2 o v e r f i r e a i r mass flow rate and v -the o v e r f i r e a i r nozzle o v e l o c i t y . Thus the production of small scale turbulence by o v e r f i r e a i r j e t s depends on the nozzle v e l o c i t y and the o v e r f i r e a i r flow rate. In the furnace, i t i s desirable f o r much of the small scale turbulence to be produced at the l e v e l of the o v e r f i r e a i r nozzles, thus concentrating the mixing e f f e c t of the turbulence near the f u e l grate. Intense mixing near the f u e l grate reduces the residence time required f o r gas phase combustion, and concentrates combustion near the f u e l p i l e , drying the f u e l and page 71 minimizing the under grate a i r required f o r stable combustion. In the model, measurements were made of u' and w', which are the standard deviations of the h o r i z o n t a l and v e r t i c a l v e l o c i t y components. Average turbulence i n t e n s i t y , v'= ^(u'+w') i s presented i n fig u r e 8.2.7 f o r the plane Y«=8.1 f t . , f o r case 1 (150k lb/hr, 60% o v e r f i r e a i r ) . There i s a d i s t i n c t region of high turbulence i n the o v e r f i r e a i r j e t s . This region extends from the furnace side walls, to the plane of symmetry. I t i s apparent that the o v e r f i r e a i r j e t s , i n t h i s case, provide a region of high turbulence throughout the furnace cross section, at the l e v e l of the o v e r f i r e a i r j e t s . A s i m i l a r r e s u l t occurs i n the plane Y=14.4 f t . , case 1, i n Appendix E.4.3. page 72 t i 1 1 < 1 1 1 S 1 1 1 1 i « [ J : j ) m a ! i o • » < « • r < ' ^ ' 1 INTENSITY , F T / 5 m - :>100 :>80 , <100 :>60 , <80 :>40 , <60 :>20 , <40 :>0, <20 B -B >= O -• •= 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , FEET FROM FURNACE CENTER L I N E Figure 8.2.7 Average Turbulence Intensity v' i n the Plane Y=8.1 f t page 73 Unfortunately however, scales of turbulence are not known. As discussed i n section 7.3, o s c i l l a t i o n of the o v e r f i r e a i r j e t s has been observed i n the model. Therefore, o s c i l l a t i o n of structures such as j e t s and r e c i r c u l a t i o n zones undoubtedly contributes to v', even though the co n t r i b u t i o n to mixing i s very small. Since the bank of j e t s on each side of the plane of symmetry o s c i l l a t e with respect to each other, only i n the mean flow i s U equal to zero i n the plane of symmetry. As the j e t s o s c i l l a t e , the d i r e c t i o n of U reverses at any point that i s near both the plane of symmetry and the o v e r f i r e a i r j e t s . The peak values of U could approach the ce n t e r l i n e v e l o c i t y of an unconfined j e t , because when the j e t s on e i t h e r side of the plane of symmetry are not c o l l i d i n g head on, f o r an instant, the j e t s are unconfined. Since U i s much greater than W i n an unconfined, h o r i z o n t a l j e t , the co n t r i b u t i o n to u' by the o s c i l l a t i o n of the j e t s would be much greater than the co n t r i b u t i o n to w', near the plane of symmetry. u' and w' both contain small scale turbulence as w e l l , which promotes micro scale mixing. u' The isotropy r a t i o R (defined In se c t i o n 7.1 as — , ) • ^ s p l o t t e d f o r case 1, Y-8.1 f t . i n figure 8.2.8. Near the plane of symmetry, u' i s at l e a s t 1.5 times greater than w'. A s i m i l a r r e s u l t can be found i n the plane Y-15 f t . i n Appendix E.5.2. I f i t i s assumed that small scale turbulence i s nearly i s o t r o p i c , i t can be concluded that the differ e n c e between u' and w' i s mostly due to the o s c i l l a t i o n of the j e t s , w' consists of small scale, near page 74 i s o t r o p i c turbulence with a small, though unknown cont r i b u t i o n from the o s c i l l a t i o n of the j e t s . Since u' > 1.5w' at l e a s t 2/3 of u' i s due to the o s c i l l a t i o n of the o v e r f i r e a i r j e t s . Therefore, near the plane of symmetry, i n the o v e r f i r e a i r j e t s , w' i s a bett e r i n d i c a t o r of micro scale mixing than u' or v'. o -<r -o to- • o CM- • page 75 o o- • o CO' CO CO o or LJ LJ t\ O o LJ _ N ° O in' LJ > ^ o ^ t— o LJ LJ 0 0 ^ o rt • M o o' i o i to i i o i r 0 i 9 0 i ; o 0 0 ( t > 1 i i > < < 1 1 S 1 s < < 6 > o > 8 o L J L * 6 : » 0 o g o e r „. j . . . _ > 9 9 9 a «? 1 « - H , ) e 9 9 = :.667<R<1.5 = :R>1.5 = :R<.667 T 1 1 1 1 i i r •1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , FEET FROM BOILER CENTER LINE Figure 8.2.8 Isotropy Ratio R, Case 1, i n the Plane Y=8.1 f t page 76 V e r t i c a l turbulence i n t e n s i t y , w' i s p l o t t e d i n the plane Y=8.1 f t . , case 1, i n figu r e 8.2.9. I t i s apparent from the fi g u r e , that intense turbulence i s produced i n the o v e r f i r e a i r j e t s , but not near the plane of symmetry. In the regions of the flow away from the plane of symmetry, the turbulence i s cl o s e r to being i s o t r o p i c . Therefore the use of v' as an i n d i c a t o r of small scale turbulence and micro scale mixing i s j u s t i f i e d , away from the plane of symmetry. Average turbulence i n t e n s i t y , v' i n the plane Y°=2.5 f t . , case 1, i s p l o t t e d i n Appendix E.4.1. v' i s low compared to the region of the j e t s i n the planes Y-8.1 f t , and Y-14.4 f t ; and i t i s quite uniform. There i s no region of intense turbulence. The plane Y=2.5 f t . i s located between the front o v e r f i r e a i r nozzle, and the fro n t w a l l . C l e a r l y , the o v e r f i r e a i r j e t s cause l i t t l e micro scale mixing near the front w a l l . Figure 8.2.10 i s a p l o t of v' i n the plane Z-.065 f t , case 1, which i s s l i g h t l y above the center l i n e s of the o v e r f i r e a i r nozzles. The fi g u r e can be divided into two d i s t i n c t regions: one of low turbulence i n t e n s i t y , between Y-5 f t . , and the fr o n t wall, and the other, of high turbulence i n t e n s i t y , from Y-5 f t . to the rear w a l l . I t i s apparent that turbulence i s enhanced by the o v e r f i r e a i r j e t s , only i n the region into which they extend. Thus, the 5.6 foot space between the front o v e r f i r e a i r nozzle and the f r o n t w a l l r e s u l t s i n poor micro scale mixing i n the v i c i n i t y of the fr o n t w a l l . page 77 9 t 1 9 ; » 9 > 6 t r i i 4 > < t . i > 1 ^ ! J * ) • a I I i l i S E • * t 1 I V 1 1 1 1 INTENSITY, FT/5 • - :>100 :>80, <100 :>60, <80 :>40, <60 :>20, <40 :>0, <20 B -B «= O -a «= 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X, FEET FROM FURNACE CENTER LINE Figure 8.2.9 V e r t i c a l Turbulence In t e n s i t y w', Case 1, i n the Plane Y=8.1 f t page 78 o t \ -——i o CD • O in • o CE o S o O ~ -or ~ L _ o L x J 2 C E o § » • CD O o I —1 ° L J CD' LxJ L _ o o o CM o o . t • • • •: J » 0 • E 1 i ! i i * * m - - •I * • JL r s • f l L L i * .. * * * a i • • • f " T ; • • V 9 * * • L ^_ ^ ^ 1 i 1 i ! - * • i 1 c • • i 1 1 1 i k 1 i r • i i i ; ^ 4 : 1 6 e • i 0 i i • • • • 9 ' r • • • INTENSITY, FT/S • - :>100 :>80, <100 :>60, <80 :>40, <60 :>20, <40 :>0, <20 a -B = o «= •1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X, FEET FROM FURNACE CENTER LINE Figure 8.2.10 Average Turbulence Intensity v' Case 1 in the Plane Z=.07 ft page 79 In Appendices E.4.6 and E.4.7, v', case 1 i s p l o t t e d i n the h o r i z o n t a l planes Z=1.625 f t . and Z=ll f t . In both cases, v' i s lower near the front wall than i t i s near the rear of the furnace, at the same l e v e l . The e f f e c t of the 5.6 foot space between the front o v e r f i r e a i r nozzles and the front wall influences the turbulence i n t e n s i t y w ell above the nozzles. Average turbulence i n t e n s i t y , v' i s p l o t t e d i n the v e r t i c a l planes Y=8.1 f t . and Y-14.4 f t . f o r cases 2 and 3 i n Appendices E.4.10 to E.4.14. No region of high turbulence i s observed, except near to the o v e r f i r e a i r nozzles. v' i s p l o t t e d i n the h o r i z o n t a l plane Z--4.46 f t . f o r case 1 and case 4 i n Appendices E.4.15 and E.4.4. v' i s quite uniform i n both cases. Therefore, the non uniform v e l o c i t y p r o f i l e above the grate, discussed i n s e c t i o n 8.1 appears to have l i t t l e e f f e c t on the turbulence i n t e n s i t y p r o f i l e , and the space between the front o v e r f i r e a i r nozzles and the fr o n t wall has l i t t l e e f f e c t on the d i s t r i b u t i o n of turbulence i n t e n s i t y above the grate. I t can be concluded that i n case 1 (150k lb/hr, 60% o v e r f i r e a i r ) , o v e r f i r e a i r j e t s increase turbulence i n t e n s i t y s i g n i f i c a n t l y at the l e v e l of the o v e r f i r e a i r nozzles producing a c u r t a i n of turbulence through which gases r i s i n g from the grate pass. Much of the f l u c t u a t i o n i n the v e l o c i t y near the plane of symmetry i n t h i s c u r t a i n however, i s due to the o s c i l l a t i o n of the o v e r f i r e a i r j e t s , and does not contribute to micro scale mixing at the l e v e l of the o v e r f i r e a i r nozzles. In a l l other cases, the o v e r f i r e a i r j e t s do not produce a region of intense turbulence. page 80 The c u r t a i n of high turbulence formed by the o v e r f i r e a i r j e t s does not extend to the front wall of the furnace because of the 5.6 foot space between the front o v e r f i r e a i r nozzles, and the front w a l l . General Conclusions on Mixing (i) The effectiveness of o v e r f i r e a i r j e t s f o r macro and micro scale mixing f o r cases 1, 2 and 3 are summarized i n table 8.2.1. The table includes the average nozzle v e l o c i t y vo, the o v e r f i r e a i r flow rate and q u a l i t a t i v e ratings of the uniformity of v e l o c i t y p r o f i l e s at the furnace e x i t , macro mixing of gases, and micro scale mixing of gases. (ii) Macro scale mixing of gases i s enhanced by o v e r f i r e a i r j e t s f o r nozzle v e l o c i t i e s greater than about 300 f t . / s . as i n cases 1 or 3, and I t i s poor f o r nozzle v e l o c i t i e s of 130 f t / s or l e s s , as i n case 2. Micro scale mixing i n the combustion zone i s enhanced s i g n i f i c a n t l y by the o v e r f i r e a i r j e t s i n case 1 only, even though Vo case 1 (357 f t / s ) i s of the same order as Vo i n case 3 (302 f t / s ) . Because the o v e r f i r e a i r flow rate of case 1 i s much higher than i t i s i n case 3, the energy a v a i l a b l e to produce turbulence i s much higher i n case 1 than i t i s i n case 3. For nozzle v e l o c i t i e s l e s s than 130 f t . / s , the v e l o c i t y p r o f i l e i n the d i r e c t i o n of the X axis, near the e x i t of the furnace, i s highly non uniform. (iii) The space between the front o v e r f i r e a i r nozzle and the fro n t w a l l of the furnace, and the non uniform v e l o c i t y p r o f i l e above the grate, r e s u l t i n a v e r t i c a l s t r a t i f i c a t i o n of the flow. page 81 Near the f r o n t wall, the oxygen concentration i s l i k e l y to be lower than i t i s away from the front wall, above the o v e r f i r e a i r nozzles. At high o v e r f i r e a i r settings, a zone of low turbulence i n t e n s i t y occurs near the front wall, while the turbulence i n t e n s i t y i n the v i c i n i t y of the o v e r f i r e a i r j e t s i s high. Mixing regions i n the furnace are shown schematically i n fi g u r e 8.2.11. General Recommendations on Mixing (i) Implement the recommendations of se c t i o n 8.1. di) Dampers should be closed to selected nozzles at low o v e r f i r e a i r flow rates, i n order to maintain high v e l o c i t y i n the r e s t of the nozzles so that j e t penetration and macro scale mixing are maintained. Table 8.2.1 Overfire A i r Jets and Mixing case 1 2 3 f t 3 O.F.A. 1000 hr 4496 1683 1683 f t v — 0 s 357 133 302 macro mixing of gases good good poor micro mixing of gases good l i t t l e c o n t r i b u t i o n from O.F.A. j e t s uniformity of v e l o c i t y p r o f i l e s near furnace e x i t good poor eg H (D 00 N> pa n> OP H« o 3 ca o s OP 3 Pi O a> OVERFIRE AIR NOZZLES HIGH OXYGEN CONCENTRATION HIGH VERTICAL VELOCITY LOW OXYGEN CONCENTRATION OV VERTICAL VELOCITY GAS RISING FROM GRATE NOT MIXED WITH OVERFIRE AIR IN THIS REGION GAS RISING FROM THE GRATE MIXES WITH OVERFIRE AIR NEAR THE GRATE LOW RATIO DF OVERFIRE AIR TO UNDER GRATE AIR GAS RISING FROM GRATE MIXES V/ITH 3VERFIRE AIR M THIS REGION HIGH OXYGEN CONCENTRATION 1VERFIRE AIR NOZZLES oo eo eo oo oo UNDER GRATE PLENUM REGIONS OF MIXING IN THE Y-Z PLANE HIGH RATIO OF OVERFIRE AIR TO UNDER GRATE AIR TURBULENCE INTENSITY IS HUCH LOVER HERE THAN IT IS IN REGION OF OVERFIRE AIR NOZZLES -IN THIS REGION GAS RISING FROM THE GRATE DOES NOT MIX V/ITH THE OVERFIRE AIR LOW OXYGEN CONCENTRATION P> OP a> 00 page 83 8.3 Aerodynamics In t h i s s e c t i o n o v e r f i r e a i r j e t models, r e c i r c u l a t i o n zones and channeling i n the flow, which are discussed i n Appendix C.5, are addressed with the r e s u l t s . J e t Models In t h i s section, the accuracy of the j e t models presented i n Appendix C. 5 when applied to the o v e r f i r e a i r system of the Woodfibre #4 power b o i l e r , i s tested. J e t c e n t e r l i n e v e l o c i t y p r o f i l e s and c e n t e r l i n e speeds are p l o t t e d i n figures 8.3.1 and 8.3.2 f o r case 1 (150k lb/hr, 60% o v e r f i r e a i r ) as described i n s e c t i o n 9.1. The curves of predicted c e n t e r l i n e t r a j e c t o r y and speed p l o t t e d on the f i g u r e s are models of e i t h e r s i n g l e round j e t s , or two dimensional s l o t j e t s i n unconfined flow. The model f o r the displacement of the j e t c e n t e r l i n e assumes uniform cross flow, and the models f o r c e n t e r l i n e speed assume no cross flow at a l l . Figures 8.3.1 and 8.3.2 are of the v e r t i c a l planes Y-8.1 f t . and Y-14.4 f t . r e s p e c t i v e l y . Figure 8.3.1 J e t Trajectory and Centerline Speed, Case 1, i n the Plane Y=8.1 f t k N i I V \ . i i y ; y 4 4 > a -o -ROUND--JET LOCUS OF MAXIMUM SPEED SINGLE ROUND NOZZLE EXPERIMENT SLOT J E T 5=2 T s 9.5 8.5 7.5 6.5 HORIZONTAL D — r . 5.5 « STANCE * = ? 1 T C D u U C D L J o or O L J ° . O o ,5 3.5 2.5 1.5 0.5 FROM NOZZLES. FT Figure 8.3.2 J e t Trajectory and Centerline Speed, Case 1, i n the Plane Y-14.4 f t page 86 I t should be emphasized that the o v e r f i r e a i r j e t nozzles on each side wall of the furnace are i n a row so that the j e t s i s s u i n g from each nozzle coalesce to some extent. Plots of U, along the d i r e c t i o n of the Y axis at Z=».065 f t . , which i s near the height of the o v e r f i r e a i r nozzle center l i n e s , are i n Appendices E.2.1 to E.2.5 f o r various values of X. At X-7.25 f t . , and X-5.375 f t . , there i s a d i s t i n c t r i p p l e i n the p l o t of U, corresponding to each set of one h o r i z o n t a l nozzle, and one downwards angled nozzle. At X-3.5 f t . , (X'=4 f t . ) , however, the r i p p l e s are much le s s d i s t i n c t , i n d i c a t i n g that four feet away from the nozzles, the i n d i v i d u a l o v e r f i r e a i r j e t s coalesce into one oblong j e t . Figures 8.3.1 and 8.3.2 are a c t u a l l y planar s l i c e s of the row of j e t s i s s u i n g from the row of nozzles on one side of the furnace. The locus of cente r l i n e s of planar s l i c e s of the row of o v e r f i r e a i r j e t s on one side of the furnace i s a c t u a l l y a surface i n three dimensions, as i s the locus of maximum speed, which represents the j e t c e n t e r l i n e i n the models p l o t t e d i n the fi g u r e s . The plane of f i g u r e 8.3.1 i s located near the center of a h o r i z o n t a l o v e r f i r e a i r nozzle. The plane of f i g u r e 8.3.2 i s i n between a h o r i z o n t a l nozzle, and a downwards angled nozzle. In f i g u r e 8.3.1, the measured locus of maximum speed ('LOCUS OF MAXIMUM SPEED' on the fi g u r e s , s e c t i o n 7.1) appears to be i n the j e t up to X'=6.9 f t . only. Beyond that, i t appears to be i n the r e c i r c u l a t i o n zone. In figu r e 8.3.2, the locus of maximum speed appears to l i e i n the j e t up to X'-5 f t only. AT X' greater page 87 than 5 feet, the locus of maximum speed goes into the r e c i r c u l a t i o n zone. Also, i n figu r e 8.3.2, near X'**0, the locus of maximum speed appears to be above the j e t . In the furnace, the bank of j e t s on one side, i s confined by the bank of j e t s on the other side of the plane of symmetry, so that the average h o r i z o n t a l v e l o c i t y , U, at any point on the plane of symmetry i s equal to 0. In the plane of symmetry i s a curve where W equals 0, i n figures 8.3.1 and 8.3.2, since the r e c i r c u l a t i o n zones extend to the plane of symmetry. Above t h i s curve, gas from the j e t forms a stream that moves upwards, and below the curve, i s a stream of gas that flows downwards. Since the speed i s low near the curve i n which W-0, the locus of maximum speed near the plane of symmetry, diverges away from the j e t to higher speeds i n the r e c i r c u l a t i o n zone. The experimental center l i n e speed i n figures 8.3.1 and 8.3.2 therefore, does not go to zero near the plane of symmetry, since the c e n t e r l i n e i s taken to be the locus of maximum speed. Since the plane of figu r e 8.3.2 i s i n between a h o r i z o n t a l nozzle, and a downwards angled one, near X'-O, i t i s a c t u a l l y between two j e t s . One j e t i s h o r i z o n t a l and the other i s angled downwards. Therefore, the locus of maximum speed i s influenced by the two non p a r a l l e l j e t s so that i t does not appear to follow the ce n t e r l i n e near X'-0. Near the nozzles, entrainment into the j e t s draws gas downwards from above the nozzles, and upwards below the nozzles. The combination of the gas streams formed by the c o l l i s i o n of the page 88 j e t s at the plane of symmetry, and the entrainment of gas at the nozzles, produces two r e c i r c u l a t i o n zones on each side of the plane of symmetry; one above the o v e r f i r e a i r j e t s , and one below them. An appropriate d e f i n i t i o n of the j e t c e n t e r l i n e i n confined flow therefore, may be the surface that divides the upper and lower r e c i r c u l a t i o n zones, rather than the locus of maximum speed. The round j e t and s l o t j e t predictions ('ROUND JET' and 'SLOT JET' r e s p e c t i v e l y In the f i g u r e s , s e c t i o n 7.1) f o r c e n t e r l i n e speed are much higher than the experimental measurements ('EXPERIMENT' i n the f i g u r e s , section 7.1). Near the plane of symmetry, the predicted v e l o c i t i e s are almost twice the measured v e l o c i t i e s . The predictions of the s l o t j e t model are somewhat less than the p r e d i c t i o n s of the round j e t model, though they appear to converge near the plane of symmetry. The predicted c e n t e r l i n e speeds are greater than the measured ones f o r the following reasons: The o v e r f i r e a i r j e t s cannot penetrate past the plane of symmetry. Therefore, the c e n t e r l i n e v e l o c i t y decays much f a s t e r i n the furnace than i t does i n unconfined j e t s . Cross flow d i s t o r t s the p r o f i l e s of the j e t s as discussed i n Appendix C.5.1. The d i s t o r t e d v e l o c i t y p r o f i l e r e s u l t s i n increased entrainment into the j e t , and greater decay of v e l o c i t y . O v e r f i r e a i r j e t s coalesce at about X'-4 f t . , so that near the nozzles, the o v e r f i r e a i r j e t s are s i m i l a r to a row of i n d i v i d u a l j e t s , and f o r X' greater than 4 feet, page 89 the j e t s are more s i m i l a r to a two dimensional s l o t j e t . Thus, neither the model of a s i n g l e round j e t , nor a s l o t j e t , are completely adequate. From the p l o t t e d v e l o c i t y vectors i n figures 8.3.1 and 8.3.2, i t can be observed that the o v e r f i r e a i r j e t i s d e f l e c t e d upwards i n the plane Y=8.1 f t . and downwards i n the plane Y=14.4 f t . The predicted j e t c e n t e r l i n e ('SINGLE ROUND NOZZLE' on the figures, s e c t i o n 7.1), however, i s deflected upwards. Also, the predicted j e t c e n t e r l i n e i s quite d i f f e r e n t from the experimentally measured locus of maximum speed. The discrepancies are explained as follows: The predicted c e n t e r l i n e i s based on a model of the locus of maximum speed i n a j e t i s s u i n g from a round nozzle into and unconfined, uniform crossflow. I t has been demonstrated i n t h i s s e c t i o n that the locus of maximum speed does not represent the t r a j e c t o r y of a j e t i n confined crossflow, very w e l l . Also, since the j e t i n the furnace, cannot penetrate past the plane of symmetry, i t may be d e f l e c t e d more than the unconfined j e t with the same cross flow v e l o c i t y . As has been discussed i n t h i s section, a row of o v e r f i r e a i r j e t s i s not l i k e a s i n g l e round j e t , or a two dimensional plane j e t . Ivanov (Appendix C.5) demonstrated that c l o s e l y spaced j e t s are d e f l e c t e d more, i n crossflow, than a s i n g l e round j e t . The cross flow v e l o c i t y i s not uniform. As demonstrated page 90 i n s e c t i o n 8.1, the v e r t i c a l component of v e l o c i t y above the grate i s much higher near the front w a l l than over the r e s t of the grate, even when there i s no o v e r f i r e a i r . The non uniform v e l o c i t y p r o f i l e above the grate i n t e r a c t s with the r e c i r c u l a t i o n zones of the o v e r f i r e a i r j e t s and the 5.6 foot space between the f r o n t wall and the f r o n t o v e r f i r e a i r nozzles. The i n t e r a c t i o n produces a downwards crossflow v e l o c i t y near the rear of the furnace, and a very high v e r t i c a l crossflow near the f r o n t . Thus the o v e r f i r e a i r j e t i n the plane Y-8.1 f t . i s d e f l e c t e d upwards, and the j e t at Y=14.4 f t . i s d e f l e c t e d downwards. J e t t r a j e c t o r i e s and c e n t e r l i n e speeds are p l o t t e d f o r cases 2 (107k lb/hr, 31% o v e r f i r e a i r , a l l dampers open) and 3 (107k lb/hr. 31% o v e r f i r e a i r , dampers closed to h o r i z o n t a l nozzles) i n Appendices E.6.3 to E.6.8. The r e s u l t s are s i m i l a r to those of case 1. Comparison of the vector p l o t s of the flow f i e l d i n Appendix E.6 to the photos of smoke i n the model flow f i e l d (figures 8.2.2, 8.2.3 and 8.2.4 and photographs not included i n the present t h e s i s ) , demonstrates that the p o s i t i o n of the j e t s as revealed by smoke flow v i s u a l i z a t i o n i s consistent with the flow f i e l d as measured by the pulse wire anemometer. The throw, Th and penetration Lj f o r cases 1, 2 and 3 were predicted with equations C.5.1.15, C.5.1.16 and C.5.1.20. The r e s u l t s are shown i n table 8.3.1. They are a l l much greater than page 91 the width of the furnace. Table 8.3.1 Predicted Throw and Penetration case 1 2 3 load 1000 lb/hr 150 107 107 % O.F.A. 60 31 31 dampers open open closed dimensions and speeds average nozzle diameter, do f t . .471 .471 .443 s l o t width,y f t . J 0 .164 .164 .144 nozzle v e l o c i t y v f t / s o ' 357 133 302 crossflow v e l o c i t y v f t / s 8 10.1 throw and 10.3 p e n e t r a t i o n 10.3 throw Th f t 448 62 281 penetration Lj f t 39 14 31 furnace width f t 17 17 17 I t i s c l e a r that models f o r unconfined j e t s do not give good r e s u l t s i n furnaces with opposing banks of o v e r f i r e a i r j e t s , except perhaps f o r very small j e t s . As a rough r u l e of thumb, i t appears to be possible to approximate the maximum gas speed i n the v i c i n i t y of the j e t with the round or s l o t j e t models of equations C.5.1.3 and C.5.1.8, only i f the c a l c u l a t e d r e s u l t i s divided by two. The locus of maximum speed i s not a good model f o r the ce n t e r l i n e s of opposing banks of j e t s except, possibly, f o r weak j e t s . R e c i r c u l a t i o n Zones and Channeling of the Flow Adams (11) asserts that i n recovery b o i l e r s with opposing banks of o v e r f i r e a i r j e t s , j e t models i n unconfined flow are not v a l i d , as stated i n Appendix C.5.2. This i s supported by the page 92 r e s u l t s presented i n t h i s s e c t i o n f o r the hog f u e l b o i l e r . Adams suggests that flow patterns, s i m i l a r to f i g u r e C.5.2.1 occur, with r e c i r c u l a t i o n zones covering most of the furnace cross section. In f i g u r e 8.1.3, case 1, i t i s apparent that r e c i r c u l a t i o n zones cover v i r t u a l l y a l l of the furnace cross section. Channeling of the flow at the furnace center l i n e occurs. Mixing i s intense, however. At a lower o v e r f i r e a i r rate , fi g u r e 8.2.2 , case 2, reveals a flow pattern very s i m i l a r to fi g u r e C.5.2.1. As discussed i n section 8.2, the v e l o c i t y p r o f i l e near the furnace e x i t , along the d i r e c t i o n of the X axis, i s h i g h l y non uniform i n case 2, with the peak v e l o c i t y about h a l f way between the side wall and the plane of symmetry. There i s l i t t l e mixing. The channeling i s apparent i n fi g u r e 8.2.5. Thus, channeling of the flow, as described by Adams (11) occurs. However, at high o v e r f i r e a i r flow rates there i s intense mixing. In the Woodfibre #4 power b o i l e r , channeling of the flow also occurs near the f r o n t w a l l as described i n section 8.2. General Conclusions on Aerodynamics (i) The j e t models for unconfined flow do not give accurate r e s u l t s f o r flow i n furnaces with opposing rows of j e t s . (ii) The locus of maximum speed does not represent the j e t c e n t e r l i n e s adequately, i n opposing j e t s . The surface d i v i d i n g the upper and lower r e c i r c u l a t i o n zones i s more representative of the j e t c e n t e r l i n e s . (iii) The o v e r f i r e a i r j e t s coalesce into a s i n g l e , oblong j e t about h a l f way between the nozzles and the plane of symmetry. page 93 Single round j e t , or two dimensional s l o t j e t models, therefore, may not represent the flow accurately. (iv) Models f o r j e t s i n crossflow do not give accurate r e s u l t s i f the cross flow v e l o c i t y i s not uniform from the front of the grate to the rear. <v) Adams's observations i n recovery b o i l e r s , that r e c i r c u l a t i o n zones occupy most of the furnace cross s e c t i o n and that channeling of the flow occurs near the center of the furnace, apply to hog f u e l b o i l e r s . However, at high o v e r f i r e a i r flow rates, intense mixing occurs. 8.4 Buoyancy E f f e c t s When density gradients occur along a h o r i z o n t a l cross section of the furnace, buoyancy can cause areas of high v e r t i c a l v e l o c i t y (channeling) to occur as described i n Appendix C.5. In t h i s section, regions of the flow where buoyancy i s l i k e l y to cause, or reduce channeling are i d e n t i f i e d , and the e f f e c t s of buoyancy are discussed. Gas Flow Above the Grate In s e c t i o n 8.1, i t i s demonstrated that the v e l o c i t y p r o f i l e above the grate i s non uniform, with a region of high v e r t i c a l v e l o c i t y near the front. Consequently, i t i s predicted i n section 8.1 that the f u e l p i l e w i l l not cover the grate evenly. Undoubtedly, the concentrations of oxygen and combustible gases and therefore, the rate of combustion per u n i t volume of gas are not uniform above the grate. Therefore, there may be large temperature gradients i n h o r i z o n t a l planes above the grate. I t i s page 94 not c l e a r which areas above the grate would be hot and which ones would be r e l a t i v e l y cool, however. The re s u l t a n t e f f e c t of buoyancy could make the isothermal channeling near the front wall worse, or i t could attenuate i t depending on where the hot spots above the grate are. Channeling due to temperature gradients above the grate can be reduced by designing the grate and undergrate plenum to produce uniform v e l o c i t y p r o f i l e s above the grate as recommended i n sec t i o n 8.1. Over f i r e a i r j e t s that are e f f e c t i v e at macro mixing i n an isothermal flow f i e l d , as discussed i n s e c t i o n 8.2, can reduce channeling due to buoyancy. Flow Above the Overfire A i r Nozzles In s e c t i o n 8.2, i t was found that v e r t i c a l s t r a t i f i c a t i o n of the flow above the o v e r f i r e a i r nozzles occurs i n two ways:a) The 5.6 foot space between the fr o n t wall and the front o v e r f i r e a i r nozzle r e s u l t s i n a region near the f r o n t w a l l where the oxygen concentration i s l i k e l y to be r e l a t i v e l y low.(ii) When the nozzle v e l o c i t y of the o v e r f i r e a i r j e t s i s low, a region of high v e r t i c a l v e l o c i t y occurs near the plane of symmetry of the furnace. In each case h o r i z o n t a l temperature gradients undoubtedly occur because, since gases are not mixed throughout the cross s e c t i o n of the furnace, the concentration of oxygen and combustible gases, and therefore the rate of combustion per u n i t volume of gas would not be uniform throughout the furnace cross sec t i o n . Buoyancy e f f e c t s above the o v e r f i r e a i r nozzles can be page 95 reduced by p o s i t i o n i n g o v e r f i r e a i r nozzles near the fr o n t w a l l , and by s e l e c t i v e l y c l o s i n g dampers to o v e r f i r e a i r nozzles when low flow rates of o v e r f i r e a i r are used, as recommended i n section 8.2. Conclusions About Buoyancy E f f e c t s (i) Channeling of the flow may be enhanced by buoyancy e f f e c t s caused by non uniform conditions near the grate, and by channeling of the flow which are discussed i n sections 8.1 and 8.2. (ii) Channeling due to buoyancy e f f e c t s can be reduced by following the recomendations of sections 8.1 and 8.2. page 96 9 Conclusions and Recomendations For Hog Fuel B o i l e r Design and M o d i f i c a t i o n In t h i s section, conclusions and recomendations that apply i n general to hog f u e l b o i l e r s with opposing banks of o v e r f i r e a i r j e t s on the side walls are summarised. Suggestions f o r futher research are made. Conclusions About Flow Above the Grate R e c i r c u l a t i o n zones underneath the o v e r f i r e a i r j e t s cause the v e r t i c a l v e l o c i t y above the grate to be higher near the side walls than near the center of the furnace. They a f f e c t the f u e l bed as follows: (i) High v e l o c i t i e s over the grate caused by the r e c i r u l a t i o n zones w i l l e n t r a i n f i n e r f u e l p a r t i c l e s from the grate. Some of the p a r t i c l e s may be re-deposited on the f u e l bed by the downwards flow at the edges of the r e c i r c u l a t i o n zones. (ii) The non-uniform v e l o c i t y p r o f i l e above the grate probably contributes to non-uniformity of the f u e l bed thickness. (iii) Entrainment and a g i t a t i o n of p a r t i c l e s i n the f u e l bed probably improves the d i s t r i b u t i o n of oxygen to the bed, enhancing combustion i n the bed. Non uniform v e r t i c a l v e l o c i t y p r o f i l e s from the front to the rear of the furnace occur above the grate i f the following conditions occur: (i) The under grate plenum does not adequately d i f f u s e the under grate a i r flow. (ii) There i s an area of the grate near a wall of the furnace, page 97 where the open area a v a i l a b l e f o r a i r flow i s much lower than i n the r e s t of the grate. A r e c i r c u l a t i o n zone near the wall can r e s u l t . (in) There i s a large space between the front or rear wall and the nearest set of o v e r f i r e a i r nozzles. Conclusions About Mixing Ove r f i r e j e t s produce a region of high v e r t i c a l v e l o c i t y i n the middle of the furnace. R e c i r c u l a t i o n zones occur above and underneath the j e t s . The e f f e c t s of the flow f i e l d are l i s t e d below: (i) The r e c i r c u l a t i o n zones below the j e t s enhance the mixing of gases near the f u e l bed. (ii) The region of high v e r t i c a l v e l o c i t y can e n t r a i n f u e l p a r t i c l e s and e j e c t them from the furnace with short residence times. Gases i n the region of high v e r t i c a l v e l o c i t y are ejected with short residence times as w e l l . The s e v e r i t y of these e f f e c t s depends on the amount of mixing i n the furnace. (iii) The r e c i r c u l a t i o n zones above the o v e r f i r e a i r j e t s mix gases above the j e t s . Two b asic types of flow pattern occur. They can be c l a s s i f i e d according to whether the mixing that occurs as poor or good. Poor mixing occurs at low o v e r f i r e a i r nozzle v e l o c i t i e s and good mixing occurs at high o v e r f i r e a i r nozzle v e l o c i t i e s . Poor mixing i s described as follows: (i) Near the middle of the furnace, the o v e r f i r e j e t s are d e f l e c t e d f a r above the grate. Overfire a i r and gases r i s i n g from page 98 the grate have r e l a t i v e l y l i t t l e opportunity to mix and burn before e x i t i n g the furnace. (ii> V e l o c i t i e s are low i n the r e c i r c u l a t i o n zones so that they do l i t t l e to mix gases and even out differences i n excess a i r throughout the furnace cross section. Good mixing i s described as follows: (i) The o v e r f i r e a i r j e t s are d e f l e c t e d very l i t t l e . Mixing of o v e r f i r e a i r and furnace gases therefore, occurs near the grate, concentrating combustion near the f u e l bed. (ii) Large, r e l a t i v e l y high speed r e c i r c u l a t i o n zones that occur above the j e t s mix gases. P a r t i c l e s and gas i n the zone of high v e r t i c a l v e l o c i t y i n the middle of the furnace get transfered into the r e c i r c u l a t i o n zones, increasing t h e i r residence times. (iii) The j e t s o s c i l l a t e . Near the plane of symmetry, the o s c i l l a t i o n i s quite intense. This o s c i l l a t i o n may not a s s i s t mixing i n the furnace. At high o v e r f i r e a i r flow rates only, intense turbulence i s produced i n the o v e r f i r e a i r j e t s . The turbulence reduces instantaneous deviations of gas concentration from mean values, r e s u l t i n g i n reduced emissions. Poor mixing can also occur i f there i s a large space between the f r o n t or rear wall and the nearest o v e r f i r e a i r nozzles. Gases r i s i n g near the wall w i l l not mix with o v e r f i r e a i r . A region of high v e r t i c a l v e l o c i t y w i l l occur near the w a l l and the a i r to f u e l r a t i o w i l l be lower near the wall than above the o v e r f i r e a i r nozzles. page 99 Conclusions About Overfire A i r J e t Models The following conclusions apply to the use of models of j e t s i n unconfined flow f o r the case of opposing banks of o v e r f i r e a i r j e t s i n hog f u e l b o i l e r s . CD Opposing j e t s are very d i f f e r e n t from unconfined j e t s . Unlike unconfined j e t s , opposing j e t s have r e c i r c u l a t i o n zones and the c e n t e r l i n e speed of opposing j e t s decays to 0 near the plane of symmetry. (ii) I n d i v i d u a l j e t s i s s u i n g from the o v e r f i r e a i r nozzles coalesce about h a l f way between the side wall and the plane of symmetry Into an oblong j e t . Models f o r round j e t s or s l o t j e t s therefore, may not be adequate. ( i i i ) The locus of maximum speed does not represent the j e t c e n t e r l i n e well i n opposing j e t s . (iv) The j e t c e n t e r l i n e d e f l e c t i o n i s strongly influenced by non u n i f o r m i t i e s i n the v e l o c i t y p r o f i l e above the f u e l grate i f the v e l o c i t y p r o f i l e i s not uniform from the f r o n t of the furnace to the rear. (v) The c e n t e r l i n e speed of the j e t s are overpredicted by a f a c t o r i n the order of 2, near the plane of symmetry. (vi) D e f l e c t i o n of the c e n t e r l i n e i s not accurately predicted. (vii) Throw, defined as the distance from the nozzle at which a l i m i t i n g c e n t e r l i n e speed occurs, and penetration, defined as the distance from the nozzle at which the t r a j e c t o r y of the j e t approaches the v e r t i c a l d i r e c t i o n , are f a r overpredicted by the models. page 100 (viii) The models may give reasonable predictions f o r j e t s with very low nozzle v e l o c i t i e s i n uniform crossflow. (ix) The j e t penetration model of equation C.5.1.20 may be used as a r u l e of thumb guide to determine adequate j e t penetration. In table 8.3.1, Ld i s equal to about 2 furnace widths f o r case 3 i n which mixing was good. I f Lj i s s p e c i f i e d as 2 furnace widths, diameter and nozzle v e l o c i t y can be s p e c i f i e d with equation C.5.1.20. However, L j does not p r e d i c t the actual penetration of the j e t s . Recommendations (i) There should be no large, i r r e g u l a r gap i n the h o r i z o n t a l spacing of the o v e r f i r e a i r nozzles. (ii) Areas of the grate with no holes should be avoided, e s p e c i a l l y near the walls. ( i i i ) The grate and undergrate plenum should be c a r e f u l l y designed to produce a uniform v e l o c i t y p r o f i l e above the grate. The v e l o c i t y p r o f i l e may be modified by adjusting the s i z e and spacing of holes i n the grate. (iv) Dampers to o v e r f i r e a i r nozzles should be s e l e c t i v e l y closed at low o v e r f i r e a i r rates to maintain high nozzle v e l o c i t i e s i n the remaining nozzles. (v) Furnace designs should be tested with scale model studies. (vi) Fuel feeders should be designed to ensure as uniform f u e l bed as p o s s i b l e . Research Needs (i) Models f o r opposing j e t s i n crossflow would be u s e f u l to page 101 furnace designers. Since the opposing j e t s i n t e r a c t i n an unsteady, p e r i o d i c manner near the plane of symmetry, models of a s i n g l e row of j e t s , confined by a v e r t i c a l w all are not adequate. Cii) Measurement of the residence times of gas or p a r t i c l e s i n a furnace model at various r a t i o s of o v e r f i r e a i r to under grate a i r and various nozzle v e l o c i t i e s would quantify the e f f e c t s of r e c i r c u l a t i o n zones and channeling on residence times. This could be done with the present apparatus. ( i i i ) The e f f e c t of the r e c i r c u l a t i o n zones under the o v e r f i r e a i r j e t s on the f u e l p i l e i s not w e l l understood. An experiment, with no combustion, i n which the f u e l bed i s simulated with p a r t i c l e s would demonstrate the e f f e c t s . I t may be p o s s i b l e to do t h i s with the present apparatus, i f the relevent non dimensional parameters can be duplicated i n the model. (iv) An experiment with the present apparatus could be conducted i n which mixing of a tr a c e r gas i s measured would quantify mixing. Autocorrelations and cross c o r r e l a t i o n s of turbulence and concentration could be used to quantify micro mixing. (v) An experiment s i m i l a r to that of Pershing (16), as discussed i n Appendix C.4, with o v e r f i r e a i r j e t s would demonstrate the e f f e c t s of the three dimensional flow f i e l d of a furnace on p a r t i c l e entrainment and burnout. (vi) A numerical model of the Woodfibre furnace could be developed, i n c l u d i n g heat transfer, buoyancy and combustion. I t could be tested f o r the isothermal case with the r e s u l t s of the present study. page 102 BIBLIOGRAPHY 1. Lamb, T.J. et a l , "Incinerator Overfire Mixing Demonstration," Arthur D. L i t t l e Inc., Cambridge Mass., U.S. E.P.A. contract ADL 73722. EPA 600 2 75 016. EPA 68 02 0204. 162P. AUG75. 2. K h a l i l , Modeling of Furnaces and Combustors, Tunbridge, Wells, Kent: Abacus Press, 1982 3. Patterson, R.C. and Abrahamsen," Flow Modeling of Furnaces and Ducts," Combustion, v o l 33-34,pp. 47-55, 1961-62 4. Bianca J.D. and, Bauver I I , W.P., "An Aerodynamic Study of an Operating Tangentially F i r e d Furnace," F l u i d Mechanics of  Combustion Systems, presented at the F l u i d s Engineering Conference, Boulder Colo., pp. 51-63June 22-23, 1981 5. Beer, J.M.,"The Si g n i f i c a n c e of Modeling," Proceedings at the  T h i r d Symposium on Flames and Industry "The Use of Models', pp.Bl-B8 Wed., Oct. 9, 1966 6. Topley, Alex R.," The Design, Development and Operation of a Waste Wood Burning B o i l e r , " The F i f t h Annual Wood Energy Forum 81, pp.200-211, New Orleans LA, March 23-25, 1981 7. Spalding, D.B.,"The Ar t of P a r t i a l Modeling", Colloquium of Modeling P r i n c i p a l s , 9th (Int) Symposium on Combustion. Academic Press, New York pp.833-843, 1963 8. Anson, D., "Modeling Experience with Large B o i l e r s ," Proceedings of the 3rd Symposium on Flames and Industry 'The  Use of Models', pp. F1-F6, Wed., Oct. 9, 1966 9. Evans, D.G. and Patrick, M.A.," The Use of Modeling Studies i n B o i l e r Furnace I n s t a l l a t i o n s , " Journal of the I n s t i t u t e of page 103 Fuel, pp.414-420, Oct.,1966 10. Davidson, F.J." Nozzle Scaling i n Isothermal Models ," Journal  of the I n s t i t u t e of Fuel, pp.470-475, D e c , 1968 11. Adams, T.N., " A i r Jets and Mixing i n K r a f t Recovery B o i l e r s , " personal communication, Cascade Technologies Inc., Federal Way WA 96003, August, 1986 12. White F.M, F l u i d Mechanics, McGraw-Hill Book Company, New York, 1979 13. Pulsed Wire Anemometer Hardware Manual, PELA Flow Instruments Ltd., 3 Bourne Dene, Farnham, Surrey GU10 4RF, England 14. Tillman, D. Wood Combustion: P r i n c i p l e s , Processes and  Economics, Envirosphere Company, D i v i s i o n of Ebasco Company, Bellvue WA, Academic Press, 1981 15. Adams, T.N. "Research Needs i n I n d u s t r i a l Wood Combustion," personal communication, Tacoma WA, May 1987 16. Adams, T.N. "Factors a f f e c t i n g E f f i c i e n c y i n Wood F i r e d B o i l e r s , " TAPPI Proceedings, 1986 Engineering Conference, pp.67-79, 1986 17. Mohr, CM. et a l , "Incinerator Overfire Mixing Study," Arthur D. L i t t l e Inc., Cambridge Mass., U.S. E.P.A. contract ADL-72940. Aptd 1133. EPA-EHSD-71-6. 242P. Feb. 1972 18. Niessen, W.R. Combustion and Incineration Processes , M. Dekker, New York, 1978 19. Adams, T.N. "Wood-Waste Combustion and B o i l e r Improvement," personal communication, 1979 20. Simmons, W.W. and Ragland, K.W.," Single P a r t i c l e Combustion page 104 Analysis of Wood," Fundamentals of Thermochemical Biomass  Conversion, pp.777-791, 1982 21. Spurrel, R.M."Evaluating the Performance of Wood-Fired Spreader Stoker B o i l e r s , " TAPPI Proceedings, 1984 Engineering Conference, pp. 305-312, 1984 22. Kaiser, E.R. and McCaffery, J.B.,"Overfire A i r Jets f o r Incinerator Smoke Control," Combustion, pp. 20-22, August 1970 23. Junge," B o i l e r s F i r e d with Wood and Bark Residues", Research B u l l e t i n 17, Forest Products Research Laboratory, Oregon State U n i v e r s i t y , 1975 page 105 Appendix A Instrumentation In the following sections, instruments used i n the experiments are l i s t e d . C a l i b r a t i o n of the pulsed wire anemometer (p.w.a.) and o r i f i c e plates are described b r i e f l y . P r i n c i p a l sources of err o r are i d e n t i f i e d and procedures f o r obtaining accurate, repeatable measurements with the p.w.a. are l i s t e d , p.w.a. v e r t i c a l v e l o c i t y measurements are integrated across h o r i z o n l a l planes of the model to obtain flow rates. The flow rates are compared with flow rates measured with the model a i r flow system as a check of the p.w.a. r e s u l t s . The apparatus i s described i n s e c t i o n 5. A . l L i s t of Instruments The instrumentation used i n the experiments f a l l s into 3 categories, a i r flow measurement and co n t r o l , pulsed wire anemometer equipment, and smoke flow v i s u a l i z a t i o n equipment. Instrumentation i s l i s t e d i n table A.1.1. Table A.1.1 L i s t of Instruments page 106 component manufacturer d e s c r i p t i o n a i r flow measurement and c o n t r o l blower Western Blower 12 inch compressed a i r supply shop a i r 100 l b / i n2 a i r pressure regulator and Wilkerson 5-125 lb/in 2gauge f i l t e r t made i n shop 2 o r i f i c e plates f o r o v e r f i r e a i r 1 i n I.D. pipe .7 In o r i f i c e o r i f i c e p l a t e ^ f o r under grate a i r made i n shop 2 i n I.D. pipe 1.4 i n o r i f i c e p u l s e d wire anemometer equipment pulsed wire probe Pela Flow Instruments Ltd. 5/imeter sensor wires, 9/imeter pulsed wire pulsed wire Pela Flow c o n t r o l l i n g u n i t Instruments Ltd. computer with dual disk drive and p r i n t e r Commodore Super Pet 9000 32 k personal computer o s c i l l i s c o p e Techtronics dual beam analog smoke flow v i s u a l i z a t i o n smoke generator „ ^ _ . Genie Mark 4 Concept Engineering o l l m i s t s l i d e p r o j e ctor camera Yashika 35 mm S.L.R. 50 mm lens f t h i n p late o r i f i c e s designed according to ISO s p e c i f i c a t i o n White (1) A.2 C a l i b r a t i o n of Instruments The pulsed wire anemometer (p.w.a.) probe and o r i f i c e plates were c a l i b r a t e d . C a l i b r a t i o n procedures are b r i e f l y described i n t h i s s e c t i o n . The pulsed wire anemometer and the model a i r flow systems are described i n se c t i o n 5. Pulsed Wire Anemometer The p.w.a. probe was c a l i b r a t e d according to the procedures page 107 i n the manual provided by the manufacturer (2). V e l o c i t y ( i i ) , was c a l i b r a t e d against the time of f l i g h t (t) according to the following equation: A -U™ £ + ^ 3 0<|u | < | u m a x | or | u m i n | A. 2.1 The probe was placed i n a low turbulence wind tunnel with the pulsed wire perpendicular to the flow d i r e c t i o n and the p o s i t i v e sensor wire located downstream of the pulsed wire. The a i r speed u was measured with a p i t o t tube and manometer, t was measured with the p.w.a. The speeds U m a x and u m i n are the maximum a i r speeds i n which equation A.2.1 f i t s the curve of u vs t, f o r the p o s i t i v e and negative sensor wires r e s p e c t i v e l y . The constants A and B were found with a l e a s t squares technique. The probe was then rotated 180° and the procedure was repeated f o r the negative sensor wire. There i s a s i g n i f i c a n t c a l i b r a t i o n e r r o r at low speeds which i s discussed i n se c t i o n A.3. O r i f i c e Plates The undergrate and o v e r f i r e a i r flow rates i n the model were measured with o r i f i c e plates as discussed i n se c t i o n 5.2. A l l o r i f i c e p l a t e s were designed to conform to the dimensions of an ISO standard t h i n plate o r i f i c e (12). The diameters of the o r i f i c e p l a t e s are l i s t e d i n table A.1.1. Each o r i f i c e p l ate i s located downstream of a t h i r t y diameter long pipe to insure that flow at the o r i f i c e i s f u l l y developed. The c o r r e l a t i o n recommended by ISO (12) between the pressure drop across the taps of a t h i n plate o r i f i c e and the a i r flow rate was used f o r the under grate o r i f i c e p l a t e . There was a s i g n i f i c a n t pressure drop between the o v e r f i r e page 108 a i r o r i f i c e plates and the o v e r f i r e a i r plenums i n the model so that the ISO c a l i b r a t i o n could not be used. I t was therefore necessary to measure the flow rate through each o r i f i c e p late and c a l i b r a t e i t against the pressure drop across the pressure taps. The o v e r f i r e a i r p i p i n g was disconnected from the o v e r f i r e a i r plenum and connected to a f i v e foot length of 2 inch I.D. pipe. The c e n t e r l i n e v e l o c i t y was measured with a p i t o t tube. The v e l o c i t y p r o f i l e i n the pipe was estimated with a c o r r e l a t i o n f o r turbulent pipe flow from White (12). With the c e n t e r l i n e v e l o c i t y and the v e l o c i t y p r o f i l e , the flow rate was found and c o r r e l a t e d against the pressure drop across the taps for each o r i f i c e p l a t e . A.3 P r i n c i p a l Sources of Error Error occurs i n the estimated v e l o c i t y f i e l d of the prototype furnace due to: Ignorance of combustion i n the model. Ignorance of bouyancy i n the model. D i s t o r t i o n of geometry i n the model. Erro r i n v e l o c i t y measurements i n the model. Error due to the ignorance of combustion and buoyancy, and the d i s t o r t i o n of geometry can not be qua n t i f i e d . The r e s u l t s of the isothermal model study, however, are at l e a s t q u a l i t a t i v e l y c o r r e c t . Even i f the flow f i e l d i s not modeled exactly, the r e s u l t s can be used to p r e d i c t the occurance of conditions that are good or poor f o r combustion i n the furnace. V e l o c i t y measurement error r e s u l t s from several sources, they are: E r r o r i n the c a l i b r a t i o n of the p.w.a. probe. page 109 Error due to the p r o b a b l i s t i c and o s c i l l a t o r y nature of the turbulent flow f i e l d . E rror i n the c a l i b r a t i o n of the o r i f i c e p l a t e s . V a r i a t i o n s i n the model a i r flow rates once they are set. The main source of error i n v e l o c i t y measurement at low speeds i s the c a l i b r a t i o n of the p.w.a. probe. There are two sources of er r o r i n the c a l i b r a t i o n at low speeds. As discussed i n s e c t i o n A.2, the time of f l i g h t was co r r e l a t e d to the a i r speed i n a wind tunnel which was measured with a p i t o t tube and a manometer. Error occurs because the minimum speed that the p i t o t tube and manometer could measure was 2.4 f t / s . The probe therefore, was not properly c a l i b r a t e d i n the range -2.4 f t / s < u < 2.4 f t / s . When 2.4 f t / s i s scaled up to the prototype furnace with a s c a l i n g f a c t o r of 10 to 16, as i n sec t i o n 7.1, a speed of 24 to 38 f t / s r e s u l t s . Thus the accuracy of measurement where the speed i s l e s s than 24 to 38 f t / s , at low turbulence l e v e l s , i s i n doubt. When measurements are taken where the turbulence i n t e n s i t y i s high, only a small f r a c t i o n of the v e l o c i t y samples taken at a p a r t i c u l a r l o c a t i o n may be i n the uncalibrated range. The second err o r at low speeds occurs because, i n the manometer used to c a l i b r a t e the probe, there i s a constant absolute e r r o r i n the height of the column of f l u i d of Tl/2 of one d i v i s i o n . At the lowest speed (2.4 f t / s ) the column of f l u i d i s at the f i r s t mark so that the erro r i n the height of the column of f l u i d i s T50%, which corresponds to an error i n v e l o c i t y of T25%. page 110 The r e l a t i v e e r r o r decreases as v e l o c i t y increases however. Therefore, at the speeds approaching 24 to 38 f t / s i n the prototype furnace, the error approaches T25%. This er r o r i s reduced at higher speeds. At higher speeds, the p r i n c i p a l source of erro r i n v e l o c i t y measurement i s due to the p r o b a b l i s t i c and o s c i l l i a t o r y nature of the turbulent flow. I t was necessary to take many samples f o r each v e l o c i t y measurement, over a long enough period of time to obtain meaningful average v e l o c i t i e s . A l l v e l o c i t y measurements taken, except at low v e l o c i t i e s were repeatable wi t h i n T5% 95% of the time. Procedures f o r obtaining repeatable v e l o c i t y measurements are l i s t e d i n s e c t i o n A.4. In the experiment, p.w.a. measurements were accepted at a l l traverse l o c a t i o n s , regardless of turbulence or mean v e l o c i t y . In most p l o t s of the data v e l o c i t y measurements ranged from near 0 f t / s to 20 f t / s i n the model, so that the erro r i n the measurements of small v e l o c i t i e s was small compared to the l a r g e s t v e l o c i t i e s of the p l o t . A.4 Obtaining Accurate and Repeatable V e l o c i t y Measurements w*?k__the_P.W.A. In order f o r accurate repeatable measurements of u and u' to be obtained, there must be enough samples to obtain s t a t i s t i c a l l y s i g n i f i c a n t r e s u l t s , the time over which samples are taken must be long compared to the period of any o s c i l l a t i o n s i n the flow (often i n the order of 10 seconds), the s i g n a l q u a l i t y must be good and v e l o c i t i e s must f a l l w ithin the c a l i b r a t e d range of the instrument. The p.w.a. i s described i n sec t i o n 5.3, and procedures page 111 followed are i n s e c t i o n 6.2. Instructions f o r use of the p.w.a. and r e l a t e d software are i n the manual provided by the manufacturer (13). Number of Samples The program prompts f o r the number of samples. I f i t i s assumed that v e l o c i t y i s normally d i s t r i b u t e d , and that i t i s independent of time, the number of samples required can be c a l c u l a t e d i f u and u' are known. The following expression i s from the manual provided by the manufacturer of the p.w.a. equipment jV=the number of samples required f o r measurements of u to be repeatable w i t h i n T 5%, 95% of the time. The procedure to assure that the number of samples i s greater than N, i s as follows: Take a measurement with 500 to 2000 samples. Obtain u and u' from the computer printout. Calculate M with equation A.4.1. I f JV > the number of samples already taken, repeat the measurement with at l e a s t M samples. I f JV < the number of samples already taken the values of u and u' obtained are assumed be repeatable as s p e c i f i e d . (13): N =1082 A.4.1 u' I f u i s below one foot per second or i f 2 > 6 the number of samples required to obtain the s p e c i f i e d page 112 r e p e a t a b i l i t y i s excessive, and equation A.4.1 i s ignored, and 2000 samples are taken. Unsteady Flow The time of f l i g h t can be observed with the o s c i l l i s c o p e . At sampling rates i n the order of 50 samples per second, the successive time of f l i g h t traces on the o s c i l l i s c o p e can sometimes be observed to change c y c l i c a l l y over time i n t e r v a l s as long as about 30 seconds. The flow i s therefore unsteady i n some regions. I f the flow i s observed to be, or suspected of being unsteady, the following procedure i s used to assure repeatable measurements of u and u': Measure u and u' with at l e a s t N samples. Repeat with at l e a s t N samples. I f the d i f f e r e n c e between the two measured v e l o c i t i e s i s l e s s than 5%, the measurement i s acceptable. I f the d i f f e r e n c e between the two measured v e l o c i t i e s i s greater than 5%, the time over which the v e l o c i t y i s sampled i s increased by increasing the number of samples, and keeping the time delay between samples the same as before. The measurement i s repeated with the same number of samples and the r e s u l t i n g measurements of u are compared again. I f necessary, the process i s repeated. Signal Q u a l i t y V e l o c i t y can be c a l c u l a t e d accurately from the time of f l i g h t only i f the s i g n a l q u a l i t y i s good. page 113 Spurious si g n a l s occur i n reversing flows. The ribbon of warm a i r produced during one sampling cycle may not be cleared from the area of the probe before the next sampling cy c l e . As a r e s u l t , very short times of f l i g h t are recorded. The program r e j e c t s times of f l i g h t shorter than a c u t - o f f value s p e c i f i e d by the user, and records a v e l o c i t y sample of 0. Sometimes, however, up to 1/2 of the samples have f l i g h t times le s s than the c u t - o f f value. I f more than 5% of the f l i g h t times are le s s than the c u t - o f f value, the time delay between samples i s doubled, allowing more time f o r the heated a i r to leave the area of the probe. Spurious si g n a l s also occur when the e f f e c t of turbulence on the sensor wires causes signals to be produced, stopping the counter. Spurious 0 v e l o c i t y signals occur when the s i g n a l produced i n the sensor wire by the heated a i r i s too weak to stop the counter. These problems are minimized by c a r e f u l adjustment of the c o n t r o l l e r before the probe i s c a l i b r a t e d . However, i t i s unavoidable that some of these spurious signals do occur. Range of C a l i b r a t i o n The probe i s c a l i b r a t e d w i t h i n the range umin < u < Umax. The c a l i b r a t i o n i s not v a l i d outside t h i s range. The program prompts f o r umin and Umax, and ignores a l l samples outside the range. I f , however, a s i g n i f i c a n t f r a c t i o n of the samples are outside the c a l i b r a t e d range, an accurate measurement w i l l not be obtained. I f , i n a region of high v e l o c i t y , such as occurs near the o v e r f i r e a i r nozzles, more than 2% of the samples are outside the c a l i b r a t e d range, the a i r flow rates into the model are reduced by 1/2. The r e s u l t i n g measurements of v e l o c i t y are then m u l t i p l i e d by page 114 two so that they are scaled to the other model v e l o c i t y measurements. This assumes that only the r e l a t i v e v e l o c i t i e s are important, and not the absolute values as discussed i n sec t i o n 4.3. The c a l i b r a t i o n of the probe i s not accurate near 0 v e l o c i t y unless the turbulence i n t e n s i t y i s high. The O.F.A. and G.G.F. flow rates should therefore be set as high as poss i b l e f o r measurements i n areas where the v e l o c i t y i s low. App.endixA. 5 Integration of P.W.A. Results to Obtain Model Flow Rates Flow rates through h o r i z o n t a l cross sections of the model were c a l c u l a t e d by in t e g r a t i n g the v e r t i c a l v e l o c i t i e s measured with the P.W.A. The r e s u l t i n g c a l c u l a t e d flow rates are compared with the a i r flow rates measured with the model a i r f l o w system (described i n se c t i o n 5.2) as a check of the accuracy of the p.w.a. r e s u l t s . The region of v e l o c i t y measurement i s shown i n f i g u r e 5.3.3. In the X d i r e c t i o n (as defined i n f i g u r e 5.3.3), v e l o c i t i e s were measured i n 1.5 inch increments over a t o t a l distance of 6.8 inches. There i s a space of 1 inch between the side w a l l and the nearest v e l o c i t y measurement l o c a t i o n s . V e l o c i t i e s at the walls were assumed to equal the v e l o c i t i e s at the nearest measurement l o c a t i o n s . Since the g r i d i s coarse, and since the v e l o c i t i e s near the walls are not known, a great deal of accuracy can not be expected. page 115 Table A. 5.1 Flow Rates Calculated with the P.W.A. case 1 2 measured by model a i r f l o w system O.F.A.m f t 3 / m i n 45 17 G.G.F.m f t 3 / m i n 50 51.5 3 t o t a l f t /min 95 68.5 i n t e g r a t e d p.w.a. r e s u l t s inches above flow rates O.F.A. nozzles f t 3 / m i n -3.6 23 0.06 79 1.3 106 108 8.8 86 The c a l c u l a t e d flow rates and the flow rates measured by the model a i r f l o w system (section 5.2) are l i s t e d i n table A.5.1. A l l qua n t i t i e s i n the table are i n model u n i t s . O.F.A.m i s the model o v e r f i r e a i r volume flow rate. G.G.F.m i s the volume flow rate of a i r through the grate i n the model. The c a l c u l a t e d flow rates at 1.3 i n and 8.8 i n above the o v e r f i r e a i r nozzles should equal the t o t a l flow rate. The c a l c u l a t e d flow rate at -3.6 i n below the o v e r f i r e a i r nozzles should equal G.G.F.m. The c a l c u l a t e d flow rate at .06 i n should be between the t o t a l flow rate and G.G.F.m. There are lage errors f o r case 1, Z--3.6 i n and case 2, Z-1.3 i n . In both instances, the v e r t i c a l speed i s below the minimum c a l i b r a t e d speed ( 2.4 f t / s , Appendix A.3) over much of the cross section, and the turbulence i n t e n s i t y i s low. I f the p.w.a i s not accurate i n regions of low v e l o c i t y and turbulence, i t i s at l e a s t consistant, so that measured flow patterns are at l e a s t q u a l i t a t i v e l y c o r r e c t . page 116 Appendix B The Woodfibre #4 Power B o i l e r The following sections include a d e s c r i p t i o n of the Woodfibre #4 power b o i l e r , a d e s c r i p t i o n of the current modification, a combustion c a l c u l a t i o n and c a l c u l a t i o n of the gas flow rates and de n s i t i e s under the operating conditions simulated with the scale model. In the following sections the subscript "p" r e f e r s to the prototype furnace as opposed to the model. The subscript "o" r e f e r s to o v e r f i r e a i r , the subscript " f " r e f e r s to f l u e gas at the furnace e x i t and the subscript "g" r e f e r s to gas r i s i n g from the grate. B . l General Des c r i p t i o n of the B o i l e r The #4 power b o i l e r furnace i s a f i x e d grate spreader stoker u n i t . I t i s about 50 fee t high with an i n t e r i o r width of 17 feet and an i n t e r i o r depth of 18.7 feet . The furnace i s constructed with v e r t i c a l membrane water walls and a water cooled grate. On the fr o n t w a l l are four bunker-C o i l f i r e d burners. Hog f u e l i s de l i v e r e d by 3 chain feeders, into three hoppers on the f r o n t w a l l about two fee t above the h o r i z o n t a l plane through the center of the o v e r f i r e a i r nozzles. Windswept a i r issues from a nozzle i n each hopper to spread f u e l across the grate. The furnace can be f i r e d by bunker-C o i l , hog f u e l , or a combination of the two. When the b o i l e r was i n s t a l l e d , i t was rated at 200000 lb/hr steam at 600 p.s.i./725°F when f i r e d by o i l alone, and 150000 lb/hr steam at 600 p.s.i./725°F when f i r e d by page 117 hog f u e l alone. In November 1986, new o v e r f i r e a i r nozzles were i n s t a l l e d on the side walls as shown i n fig u r e B . l . l . There are two sets of f i v e round nozzles on each side w a l l . Each nozzle of one set i s h o r i z o n t a l and f l u s h with the wall, with a diameter of 6 inches. Each nozzle of the other set has a diameter of 5 5/16 inches and extends 16 inches into the furnace. The part of the nozzle that extends i n t o the furnace i s angled down 15° from h o r i z o n t a l . A space of 5.6 fee t was l e f t between the fr o n t w a l l and the ce n t e r l i n e s of the fr o n t o v e r f i r e a i r nozzles so that the j e t s would not i n t e r f e r e with the t r a j e c t o r y of the hog f u e l p a r t i c l e s entering the furnace from the hoppers. One t h i r d of the holes i n the grate were closed to increase the pressure at the o v e r f i r e a i r plenums. page 118 d c o i . Lw •5.6 f t — I In If ft if If Plane of Symmetry ff ff |f || || d > d Plan View Tip Angled Down 15 Degrees Fron Horizontal Detail o f Nozzles Figure B . l . l Present Overfire A i r Nozzle Configuration B.2 Combustion C a l c u l a t i o n A combustion c a l c u l a t i o n was performed f o r the furnace by H. A. Simons Ltd. The case considered was a 150000 lb/hr steam load. page 119 75% of the energy generated i s provided by the combustion of wood, and 25% of the energy generated i s provided by the combustion of o i l . A hog f u e l moisture content of 55% (wet b a s i s ) , and excess a i r l e v e l s of 50% f o r wood and 10% f o r o i l were assumed. The c a l c u l a t i o n i s i n table B.2.1. B.3 C a l c u l a t i o n of Gas Flow Rates and Densities i n the Furnace The prototype o v e r f i r e a i r volume flow rate (O.F.A.p), the prototype volume flow rate of gas r i s i n g from the grate (G.G.F.p) and the d e n s i t i e s of the o v e r f i r e a i r , and gas r i s i n g from the grate (p and p , r e s p e c t i v e l y ) are c a l c u l a t e d i n t h i s section. o g They are required to c a l c u l a t e the a i r flow rates and nozzle s i z e i n the model. Four operating conditions, defined by the steam load are considered. They are 150000 lb/hr with dampers open to a l l of the o v e r f i r e a i r nozzles (case 1), 107000 lb/hr with dampers open to a l l of the o v e r f i r e a i r nozzles (case 2), 107000 lb/hr with dampers open to the downwards angled nozzles only (case 3), and 60000 lb/hr with no o v e r f i r e a i r (case 4). In a l l cases wet wood i s the only f u e l . I d e a l i z a t i o n I t i s assumed that the gases r i s i n g from the grate i s f l u e gas at a uniform temperature of 1700°F. The flow rate of gas at the grate equals the f l u e gas flow rate minus the o v e r f i r e a i r flow r a t e . I t i s therefore assumed that a l l of the water vapor and gases produced by combustion are introduced to the flow at the grate. Gases emanating from entrained p a r t i c l e s of f u e l , or from f u e l p a r t i c l e s as they f a l l to the grate are ignored. I t i s page 120 fur t h e r assumed that the o v e r f i r e a i r enters at 590°F. Gas composition and temperatures are assumed to be independent of load. Table B.2.1 Combustion C a l c u l a t i o n from H.A.Simons Ltd. BOILER COMBUSTION-Vestern Pulp Partnership Ltd., Woodflbre-150 OOOlb/h BOILER DATA Temp flue gas leaving the a i r heater, ' F - 430 A i r temp after steam a i r beat er, °t 80 Temperature of fuel,' *F " ' n :- 80 •- ~ " Temperature of ash leaving the grate -2000 Blowdoun, sootbloving as a Z Eeedwater 1.5 0 Feed water, biowdown, sootblow, steam enthalpy 188 485 0 1350 FUEL DATA FUEL #1 FUEL #2 FUEL #3 TOTALS Type Bog O i l Gas Quantity, BDlb/h 23856. 2757 0 ,CFM ' - - 0 Moisture content,! 55 0 -Excess a i r , X 50 10 15 Moisture i n comb. a l r ( l b / l b ) .013 .013 .013 Beat content, Btu/lb ,Btu/scf 8900 18750 22384 - - 1015 Fraction of fu e l .89640 .10360 0 1.00 ULTIMATE ANALYSIS—BY FUEL Z02 Reqd - J -:-102 -Reqd X02 Reqd C 49.70 132.42 87.26 232.50 70.87 188.82 H2 6.00 47.62 10.94 86.83 22.98 182.41 02 43.20 -43.20 .64 -.64 .00 .00 S .00 .00 .84 .84 .00 .00 N .10 .28 6.15 Ash 1.00 .04 .00 100.00 136.84 100.00 319.52 100.00 371.23 AIR-BY FUEL Theoretical, Actual, lb/B 5.9239 13.832 16.071 8.8858 15.215 18.481 COMBUSTION PRODDCTS-BY FUEL lb/lbBD CF/BD1 lb/lbBD CF/BD1 lb/lbBD CF/BD1 C02 1.82 '26.89 3.70 47.22 2.60 38.35 S02 .00 .00 .02 .17 .00 .00 H20 , P I „ <-•» 1.87 67.60 1.18 42.40 2.29 82.76 H2 6.S3 158.-54 11.70 271.49 14.27 331.11 02 .68 13.90 .32 6.49 .56 11.31 Totals 11.21 266.93 16.41 367.77 19.72 463.53 X 02, by vol 5.21 1.76 2.44 4.85 Z 002, dry, by volume 13.49 14.51 10.07 13.60 X B20,dry, by vol&wt 20.06 33.91 7.71 13.03 13.16 21.74 18.78 Volume flue gas(wet), ACFM 106132 16900 0 123031 Height flu e gas, wet,lb/h 267516 - s -. 45252 0 312768 dry,lb/h 222811 42011 0 264822 a i r supplied,dry,lb/h 211984 41951 0 253935 BOILER HEAT BALANCE Unb'd carbon i n tot ash,Z 66.67 0 .00 Fly ash as Z of t o t a l ash + C i n ash 100.00 .00 .00 Loss due to unburned carbon 3.28 .00 : .00 Loss due to dry flue gas 9.00 . . ... 6.97 .00 Loss due to sensible heat i n ash .05 .00 .00 Loss due to H i n fu e l and H20 evapn. 23.86 6.41 11.16 Loss due to Mfgrs margin and unscc 2.00 2.00 2.00 Total heat losses 38.19 15.38 13.16 Thermal e f f i c i e n c y 61.81 84.62 .00 64.17 STEAM BALANCE Beat to steam, MBtu/h 1.31e8 4."37e7 0 l.75e8 Steam generated, lb/h 112500 37500 O 150000 Steam by type of f u e l , Z .75000 .25000 Steam from f u e l , lb/BDlb 4.7157 13.601 0 Grate Beat Release, Btu/ft2 0 page 121 Flue Gas Composition The f l u e gas composition must be estimated to c a l c u l a t e the f l u e gas density. Complete combustion i s assumed. Mole f r a c t i o n s and mean molecular weight M are ca l c u l a t e d i n table B.3.1. The mass per u n i t mass of dry f u e l data i s from column #1 of Table B.2.1. Mf=27.3 which i s very close to Mo, the mean molecular weight of a i r which equals 28.8. Var i a t i o n s i n gas density due to changes i n composition therefore, are very small which i s important because Mg i s assumed to equal Mf. Table B.3.1 Flue Gas Composition species M mass species per u n i t mass dry f u e l moles per u n i t mass dry f u e l mole f r a c t i o n of f l u e gas co 2 44 1.82 .0414 .101 SO 32 0. 0. 0. H 20 18 1.87 .1039 .253 N 28 6.83 .244 .594 °2 32 .68 .0213 .0518 t o t a l 11.21 .4107 1.00 ^ _ t o t a l mass per u n i t mass of dry f u e l _ 11.21 3 f t o t a l moles per mole of dry f u e l .4107 Gas Densities p and p are c a l c u l a t e d assuming the i d e a l gas law, Mg—27.3, o g Tg=1700°F, Mo=28.8, and To-590°F. P M p = T « B.3.1 * =49,750 ft-lb/slugmole-R P=l atmosphere=2116.8 l b / f t 2 T=(T F+460) R page 122 From equation B.3.1, p Q=l.168xl0~ 3 s l u g / f t ? and p =5.373x10"* s l u g / f t ? 8 C a l c u l a t i o n of gas Flow Rates Data f o r the following c a l c u l a t i o n i s from table B.2.1 column 1. Table B.2.1 i s a combustion c a l c u l a t i o n f o r the Woodfibre #4 power b o i l e r , provided by H.A. Simons Ltd. For a steam flow rate of 112500 lb/hr (3493.8 slug/hr), 6583.4 slug/hr of combustion a i r i s supplied and 8307.96 slug/hr of f l u e gas i s produced. Ri-6583.4/3493.8-1.88 R2-8307.96/112500=2.38 Ri i s the r a t i o of the combustion a i r mass flow rate to the steam mass flow rate. R2 i s the r a t i o of the f l u e gas mass flow rate to the steam mass flow rate. Ri and R2 are assumed to be independent of the steam flow rate. The r a t i o of o v e r f i r e a i r to t o t a l combustion a i r ($) i s determined according to f i g u r e B.3.1. This curve was provided by H.A. Simons Ltd. as a t y p i c a l combustion a i r d i s t r i b u t i o n curve. Since the d e n s i t i e s of the o v e r f i r e a i r and the under grate a i r are the same, the mass flow rate r a t i o i s the same as the volume flow rate r a t i o . The maximum rated load f o r the Woodfibre #4 power b o i l e r i s 150000 lb/hr steam, when f i r e d with hog f u e l . The r a t i o of o v e r f i r e a i r to combustion a i r can also be expressed by the following equation: *=Cl5MOO-- 43 B.3.2 L= b o i l e r load, steam flow rate lb/hr page 123 •—t < is a »-o f f— Lu o < UJ a. o O -I f\ IDVER-1 1 riRE | 1 IAIR j / i n . 1 1 M 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 ! 1 1 1 1 1 1 AIR 1 1 1 1 1 N 1 UNDERGRATE 1 1 1 1 1 1 1 II 1 I 1 1 I I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ^^^L 1 1 i i i i i • i • 40 100 PERCENT DF FULL STEAM LOAD Figure B .3 .1 D i s t r i b u t i o n of Combustion A i r The gas flow rates are c a l c u l a t e d as follows: The f l u e gas mass flow rate: mf=R2XL slug/hr B. 3 . 3 L=furnace steam load slug/hr The t o t a l combustion a i r mass flow rate: mc=RixL slug/hr B .3 .4 The f r a c t i o n of combustion a i r to the o v e r f i r e a i r nozzles: $ -r - L B.3 .2 150000 The o v e r f i r e a i r mass flow rate: mo=$mc slug/hr B. 3 .5 The mass flow rate of gas r i s i n g from the grate: mg»mf-mo slug/hr B .3 .6 The o v e r f i r e a i r volume flow rate: „ „ . mo _ 3 _ O.F.A.i> f t /hr P o B.3 .7 page 124 The volume flow rate of gas r i s i n g from the grate (grate gas flow r a t e ) : G.G.F.p- f t 3 / h r B.3.8 • P 8 Gas flow rates i n the prototype furnace f o r cases 1 to 4 are table B.3.2. %O.F.A.- $xl00. As an example, the en t r i e s f o r e 2, i n table B.3.2 were c a l c u l a t e d as follows: Steam flow rate, L-107128 lb/hr -3327 slug/hr. mf-2.38x3327=7918.2 slug/hr. B.3.3 mc=l.88x3327-6254.7 slug/hr. B.3.4 *"C -- 4) - - 3 1 4 - 3 1 - 4 * ° - F - A - B.3.2 mo-.314x6254.7-1964.0 slug/hr. B.3.5 m8=7918.2-1964.0-5953.0 slug/hr. B.3.6 O.F.A.p-1964.0/1.168xl0"3-1682915 f t 3 / h r B.3.7 G.G.F.p-5953.0/5.373xl0"*-11080173 f t 3 / h r B.3.8 page 125 Table B.3.2 Flow Rates i n the Prototype Furnace case 1 2 3 4 o p e r a t i n g c o n d i t i o n load 1000 lb/hr 150 107 107 60 load 1000 slug/hr 4.66 3.32 3.32 1.86 dampers to h o r i z o n t a l nozzles open open closed %0.F.A. 60 31.4 31.4 0 gas f1on r a t e s mf 1000 slug/hr 11.1 7.92 7.92 4.43 i c 1000 slug/hr 8.75 6.25 6.25 3.50 mo 1000 slug/hr 5.25 1.97 1.97 0 mg 1000 slug/hr 5.83 5.95 5.95 4.43 f t 3 0.F.A.p 1000 i i 4496 1683 1683 - 0 f t 3 G.G.F.p 1000 i i hr 10857 11080 11080 8254 page 126 Appendix C Survey of Theory of Combustion i n Hog Fuel B o i l e r s Combustion i n a hog f u e l b o i l e r i s an i n t e r a c t i o n of aerodynamics, heat and mass transfer, and combustion chemistry. In t h i s section, hog f u e l b o i l e r combustion i s divided into f i v e i n t e r r e l a t e d processes, each discussed i n i t s own sub section. The processes are: 1 Combustion of Wet Wood and Bark 2 Combustion of Fuel on the Grate 3 Gaseous Combustion 4 Combustion of Entrained Fuel P a r t i c l e s 5 Aerodynamics page 127 C . l Combustion of Wet Wood and Bark Combustion of hog f u e l involves drying, v o l i t i z a t i o n and oxidation. Tillman (14) suggests four basic stages: heating and drying s o l i d p a r t i c l e p y r o l y s i s gas phase p y r o l y s i s and oxidation char oxidation Heating and drying of the f u e l involves heating of the dry f u e l and the moisture i n the f u e l , the release of hydroscopically bound moisture, and the evaporation of moisture. I t i s t y p i c a l f o r the sensible heating and evaporation of moisture to consume about 99% of the of the energy required f o r heating and drying (14). Therefore moisture content greatly a f f e c t s the energy required f o r heating and drying. Heating and drying consumes about 6% of the heating value of wood with a moisture content of 45%. When wood i s heated to above about 440°F, s o l i d p a r t i c l e p y r o l y s i s occurs (14). C e l l u l o s e , hemicellulose and l i g n i n break down, producing char, water, combustible gases and l i q u i d s . Gases and l i q u i d s further break down producing CO, and small amounts of other gases. When mixed with a i r at a high enough temperature, combustion occurs, producing a luminous flame. This i s the gas phase p y r o l y s i s and oxidation phase. The remaining material i s composed of char and ash. Char consists of carbon and small amounts of combustible, organic materials. The char burns with l i t t l e flame. The ash, which co n s i s t s of non organic matter, remains unburnt. The weight f r a c t i o n of f u e l converted to char decreases as page 128 temperature increases and the char y i e l d i s higher i n f u e l s with higher l i g n i n contents, such as bark (14). Under the conditions of a hog f u e l b o i l e r furnace, 85% to 90% of the bone dry mass of wood chips i s pyrolyzed to gas and, therefore, 10% to 15% of i t i s converted to char. A phenomena that i s important to combustion i s p a r t i c l e entrainment. P a r t i c l e s become entrained when they become suspended by the v e r t i c a l component of the gas flow. P a r t i c l e s can be entrained as they f a l l to the grate from the f u e l hoppers, or they can be entrained from the f u e l grate. Many of the entrained p a r t i c l e s e x i t the furnace with the exhaust gases. P a r t i c l e s that e x i t the furnace are c a l l e d carryover. These p a r t i c l e s endure various amounts of drying, d e v o l i t i z a t i o n and char combustion before they e x i t the furnace. Entrained p a r t i c l e s abrade furnace surfaces, e s p e c i a l l y tube banks, and carryover damages induced d r a f t fans. The emission of carryover i s r e s t r i c t e d by law. I t must therefore be removed from the exhaust gases with cyclones. Furthermore, e f f i c i e n c y of a b o i l e r i s reduced i f s i g n i f i c a n t amounts of combustible material e x i t s the furnace as carryover. I t i s therefore desirable to minimize the entrainment of f u e l p a r t i c l e s , and to assure that the combustible matter i n the carryover i s minimized. C.2 Combustion of Fuel on the Grate Hog f u e l burns i n a p i l e on the f u e l grate. Fuel drying, char combustion, s o l i d phase p y r o l y s i s , and some gas phase p y r o l y s i s and combustion occur i n the f u e l p i l e . Some important subjects to be considered i n the design and page 129 operation of hog f u e l b o i l e r s , that concern combustion of f u e l on the grate, are; d i s t r i b u t i o n of f u e l on the grate, entrainment of f u e l p a r t i c l e s from the grate, d i s t r i b u t i o n of a i r to the f u e l on the grate, and the requirements f o r stable combustion of f u e l on the grate. The d i s t r i b u t i o n of f u e l on the grate depends on the density and s i z e d i s t r i b u t i o n of the f u e l p a r t i c l e s , the rate of feed of f u e l to each f u e l hopper, the windswept a i r flow from each hopper, the aerodynamics of the gas flow f i e l d i n the furnace and the d i s t r i b u t i o n of combustion a i r through the grate. A common problem with hog f u e l b o i l e r s i s that the f u e l i s not evenly d i s t r i b u t e d across the grate (15). Uneven d i s t r i b u t i o n i s caused by gases being channeled through a small area of the . grate so that regions of high and low, or even negative v e r t i c a l v e l o c i t y occur at the grate, which prevents f u e l p a r t i c l e s from s e t t l i n g where v e r t i c a l v e l o c i t i e s are high. I t i s also caused by uneven d i s p e r s a l of p a r t i c l e s above the grate due to the aerodynamics of the furnace, the f u e l hopper feed rates or the windswept a i r flow rates. Uneven d i s t r i b u t i o n of f u e l on the grate leads to channeling of the combustion a i r to areas of the grate where the f u e l p i l e i s t h i n , when more a i r i s required where the f u e l p i l e i s t h i c k . The higher gas v e l o c i t i e s above areas where the f u e l p i l e i s t h i n tend to prevent the deposition of more f u e l i n those areas. Therefore i t i s possible that much of the under grate a i r f l o w bypasses the f u e l p i l e on the grate. V e r t i c a l gas v e l o c i t i e s increase with the under grate a i r flow rate (U.G.A.). Fuel p a r t i c l e s do not change i n s i z e or shape page 130 appreciably during drying and d e v o l i t i z a t i o n so that the density of d e v o l i t i z e d p a r t i c l e s on the grate i s only ten to f i f t e e n percent of t h e i r o r i g i n a l bone dry density (16) . Fuel p a r t i c l e s on the grate are therefore very susceptible to entrainment. The conditions under which p a r t i c l e s become entrained; and the conditions under which deposition of p a r t i c l e s onto areas of high v e r t i c a l v e l o c i t y on the grate, w i l l not occur; on average, can be determined. A motionless p a r t i c l e i s l i f t e d when the drag caused by the v e r t i c a l component of the gas v e l o c i t y equals i t s weight. For a Reynolds number greater than 144, based on p a r t i c l e s i z e , the drag c o e f f i c i e n t of a wood chip i s approximately constant. Mason (16) measured the average drag c o e f f i c i e n t s of wood chips and the following semi-empirical expression was derived: T' .36 g C.2:l T-thickness of l a r g e s t p a r t i c l e entrained p - p a r t i c l e density p p -gas density Vg—gas v e r t i c a l v e l o c i t y component g=acceleration due to g r a v i t y This expression ignores l i f t , p a r t i c l e i n e r t i a , and the e f f e c t of v e l o c i t y gradients on the p a r t i c l e , but i t does i n d i c a t e i f , on average, p a r t i c l e s w i l l be l i f t e d by the flow. page 131 I t i s t y p i c a l f o r the gas above the f u e l grate to have a density p =5.4xl0 _ 3slug/ft 3. g-32.2 f t / s e c 2 . Wet Douglas f i r 8 t y p i c a l l y has a density, p -.870 s l u g / f t 3 . The v e r t i c a l component p of gas v e l o c i t y when averaged over a v e r t i c a l cross s e c t i o n of a hog f u e l furnace, above the o v e r f i r e a i r nozzles, at f u l l power, i s t y p i c a l l y 20 f t / s e c . I f Vg=20 f t / s e c , then under the given conditions: _ 3 T- ^ 2 x 5 8 y Q 1 0 x202-.0028 f t . = .033 inches Dry, d e v o l i t i z e d wood however, has a t y p i c a l density of only 3 .087 s l u g / f t Fuel on the grate becomes dry and d e v o l i t i z e d so that the maximum thickness of p a r t i c l e s entrained from the f u e l grate i s : - 3 T- ^ 2 x 5 Q 8 7 1 0 *202=.028 ft.-.33 inches Fuel p a r t i c l e s as small as .033 inches t h i c k w i l l f a l l to the grate, but p a r t i c l e s ten times as t h i c k can be entrained from the f u e l grate a f t e r drying and d e v o l i t i z a t i o n . The U.G.A. flow rate i s therefore l i m i t e d by p a r t i c l e entrainment from the grate, and dry, d e v o l i t i z e d p a r t i c l e s must be considered. Since U.G.A. i s l i m i t e d by entrainment, combustion a i r i s provided above the f u e l grate by o v e r f i r e a i r j e t s to complete combustion of d e v o l i t i z e d gases. On areas of the grate where channeling of the flow occurs, Vg may be much .larger than 20 f t / s e c . I f , f o r example, Vg=50 f t / s e c and wet douglas f i r p a r t i c l e s are considered: page 132 - 3 T - ^ 6 2 x 5 g 4 x l ° x552-.018 ft.-.21 inches Thus p a r t i c l e s smaller than .21 inches t h i c k can not s e t t l e i n the areas of the grate i n question. Channeling of the flow at the grate must therefore be avoided. Combustion a i r i s d i s t r i b u t e d to the f u e l p i l e through small holes, t y p i c a l l y 1/2 inches i n diameter on s i x inch centers. I t i s desirable that the d i s t r i b u t i o n of a i r to the f u e l p i l e occurs such that the a i r to f u e l r a t i o i s uniform i n the gases i n and immediately above the f u e l p i l e . I f the a i r to f u e l r a t i o i s uniform, the flow rate of combustion a i r can be minimized, maximizing b o i l e r e f f i c i e n c y , and minimizing gas v e l o c i t i e s and therefore, entrainment and carryover of f u e l p a r t i c l e s . The a i r to f u e l r a t i o i s not uniform i f channeling occurs, and f u e l i s not evenly d i s t r i b u t e d on the grate. Also, Adams (15) suggests that the a i r to f u e l r a t i o i s not uniform i n the f u e l p i l e because U.G.A. i s d i s t r i b u t e d to the f u e l p i l e through small holes. High v e l o c i t y j e t s issue from the holes, e j e c t i n g p a r t i c l e s out of the f u e l p i l e and causing thinning of the f u e l p i l e i n the v i c i n i t y of each hole. The a i r to f u e l r a t i o Is probably higher near to the j e t s than between them. Since U.G.A. flow i s l i m i t e d by entrainment, combustion i n the f u e l p i l e i s sub-stoichiometric. Under sub-stoichiometric conditions, stable combustion w i l l occur only i f there i s s u f f i c i e n t oxygen present, and i f there i s enough heat radiated to the f u e l bed. Heat i s radiated from flaming gas phase combustion i n the o v e r f i r e a i r j e t s , the furnace walls, or a u x i l i a r y burners. page 133 The flow rate of U.G.A. must therefore be s u f f i c i e n t to sus t a i n stable combustion of f u e l on the grate. The amount of a i r required f o r sub-stoichiometric combustion i n the f u e l p i l e increases with f u e l moisture content. The heating and evaporation of moisture uses up energy from combustion and lowers the combustion temperature. In order to maintain a temperature high enough f o r stable combustion, more energy must be released i n the f u e l p i l e . Thus more a i r i s required i n the f u e l p i l e and less energy i s a v a i l a b l e f o r gas phase combustion above the f u e l p i l e . Mohr et a l (17) modeled combustion i n a f u e l p i l e mathematically. Their model assumed wet c e l l u l o s e and i n e r t material as a f u e l and a combustion temperature of 2000°F and that equilibrium was determined by the water gas s h i f t reaction; H20+CO?C02+H2. They c a l c u l a t e d that as the moisture content of the f u e l was Increased from 0% to 33%, the weight r a t i o of a i r to f u e l (bone dry, ash free basis) increased from 1.8 to 3.1. Less a i r was required i n the f u e l p i l e however, i f heat was added to the f u e l bed. Thus heat radiated to the bed by gas combustion, a u x i l i a r y burners or furnace walls, reduces the under grate a i r required. I t should also be noted that i f the d i s t r i b u t i o n of U.G.A. to the f u e l p i l e i s not uniform, extra U.G.A. may have to be provided to maintain stable combustion i n oxygen deprived areas of the f u e l p i l e . The following conclusions are drawn: Gas v e l o c i t i e s should be kept as low as possible, above the grate to minimize the entrainment of p a r t i c l e s of f u e l and ash. page 134 The under grate a i r flow rate, and therefore, gas v e l o c i t i e s above the grate can be minimized i f the a i r to f u e l r a t i o i s uniform throughout the f u e l p i l e . A i r i s l i k e l y to be d i s t r i b u t e d uniformly throughout the f u e l p i l e , only i f the f u e l p i l e has an even thickness over a l l of the f u e l grate. The f u e l p i l e on the grate probably w i l l not have a uniform thickness i f the v e l o c i t y p r o f i l e above the grate i s not uniform. The under grate a i r flow rate must be increased to maintain stable combustion i n the f u e l p i l e as the f u e l moisture content increases. Radiation of heat to the f u e l p i l e from gas phase combustion i n the o v e r f i r e a i r j e t s , a u x i l i a r y burners or furnace arches d r i e s the f u e l , minimizing the under grate a i r flow required. Research required i n hog f u e l bed combustion i s described as follows: Adams (15) has suggested that an experiment or mathematical modeling that couples grate hole s i z e and spacing, and p a r t i c l e s i z e d i s t r i b u t i o n with drying, d e v o l i t i z a t i o n , combustion and entrainment of f u e l p a r t i c l e s , should be conducted so that the dominant p h y s i c a l mechanisms can be i d e n t i f i e d . Radiation to the f u e l bed by furnace walls, arches and combustion i n o v e r f i r e a i r j e t s , and the aerodynamic e f f e c t s of these features on the d i s t r i b u t i o n of f u e l on the grate, and the d i s t r i b u t i o n of a i r to the f u e l , should be investigated. I f the page 135 dominant mechanisms were better understood, the p o s i t i o n of o v e r f i r e a i r j e t s and furnace geometry could be optimized so that r a d i a t i o n to the f u e l bed would be maximized and d i s t r i b u t i o n of f u e l and combustion a i r would be optimized. Reduced entrainment of f u e l p a r t i c l e s and reduced U.G.A. flow rate would r e s u l t . C.3 Combustion of Gases Since 85% to 90% of the dry mass of wood i s v o l a t i z e d i n hog f u e l b o i l e r s , combustion of gases i s of considerable importance. The gases r i s i n g from the f u e l bed consist of the products of p y r o l y s i s of the f u e l , oxygen, nitrogen, and exhaust products. The main global combustion reactions are (1): cotlo2 * co 2 2H2+02 t 2H20 Since these reactions occur very quickly (1) when the reagents are brought together into intimate contact at s u f f i c i e n t temperature, the rate of combustion i s l i m i t e d by mixing, which i s a slower process. S u f f i c i e n t oxygen i s also necessary. Thus, time, temperature, mixing and s u f f i c i e n t a i r are required f o r complete gaseous combustion. Useful concepts proposed by Lamb et a l (1) are micro scale and macro scale mixing. Macro scale mixing i s mixture i n the furnace, r e s u l t i n g i n time averaged p r o f i l e s of concentration, temperature and v e l o c i t y , that are uniform throughout the furnace cross section. Micro scale mixing i s mixture i n the furnace that reduces the Instantaneous deviations from the time averaged gas concentrations. S t i r r i n g of the gas mixture mixes i t on a macro page 136 scale. Turbulence generated by the s t i r r i n g completes the mixing on a micro scale. Micro scale mixing can be qu a n t i f i e d by the unmixedness fa c t o r used by Lamb et a l (1). In the turbulent flow of a furnace, the combustible gas concentration: C= C + c C-time averaged concentration C=»instantaneous concentration The unmixedness f a c t o r : 2 c'«= «Jc = standard deviation of f u e l gas concentration I f f u e l gas and a i r are p e r f e c t l y mixed, i . e . , c' =>0, and Cq < C < Cs (where Cq=»quenching concentration, at which the concentration i s too low f o r combustion to occur, and Cs=stoichiometric f u e l gas concentration), no f u e l gas w i l l e x i s t i n the products. As c' increases from 0, however, C can be instantaneously greater than Cs i n small packets of gas i n the flow f i e l d , r e s u l t i n g i n the emission of unburned hydrocarbons. The unmixedness f a c t o r i s reduced by j e t s i f they introduce turbulence to the furnace gases which mixes f u e l and a i r on a small scale, e l i m i n a t i n g the small packets of gas. The e f f e c t s of non-homogeneity of gases on a small scale on temperature and time averaged CO concentration are i l l u s t r a t e d by the work of Lamb et a l . The time averaged concentrations of gases and the corresponding temperatures were measured at various points in s i d e a refuse i n c i n e r a t o r . 0^ was detected on a time averaged page 137 ba s i s , at a l l points so that excess a i r was present at a l l points, when averaged over time. Figure C.3.1 i s a schematic i l l u s t r a t i o n of time averaged CO concentration vs temperature from the r e s u l t s of Lamb et a l . CO concentration was found to be high at temperatures l e s s than 1100°F, and as the temperature approached 1800°F. Temperatures l e s s than 1100°F were beli e v e d to be too low to sus t a i n combustion so that CO was not consumed and was therefore present i n the gas mixture. At points where the temperature was above 1800°F, i t was found that the amount of excess 0^ a v a i l a b l e was low, and concentration of CO was high. I t was concluded that the gases i n the furnace were not thoroughly mixed on a micro scale. % u c 8 J HOOT 1800T Gas Tenpero.'ture Figure C.3.1 Schematic of Gas Temperature vs Oxygen Concentration i n an Incinerator At any point where the CO concentration was measured, there was oxygen present on a time averaged basis, i n d i c a t i n g that C < Cs, so that a l l of the f u e l gas (CO i n t h i s case) would be page 138 consumed, i f c'-O. However, pockets of gas that were a l t e r n a t i v e l y r i c h and lean i n a i r passed by the gas sampling probe, i n d i c a t i n g that C < Cs, even though C > Cs instantaneously, some of the time, so that CO was not completely consumed. As the time averaged concentration of 0^ was reduced, temperature increased up to a maximum of 1800°F and C > Cs more often, r e s u l t i n increased emissions of CO as the temperature approached 1800°F. Gas temperature i s a f f e c t e d by heat transfer, as w e l l as excess a i r , and the e f f e c t s of heat t r a n s f e r have been ignored here. However, i f the gas mixture was homogeneous, the CO concentration would not r i s e with temperature, regardless of heat t r a n s f e r and 0^ concentration, as long as there i s excess 0^ present. Quenching occurs at about 1100°F, regardless of excess a i r . In a furnace, combustible gases r i s i n g from the f u e l p i l e must have time to mix intimately so that a l l of the combustible gases are consumed before they e x i t the furnace. The time required f o r gases emanating from the f u e l p i l e to e x i t the furnace i s c a l l e d gas residence time. I f the gases are well s t i r r e d , the time required f o r intimate micro scale mixing to occur, and therefore, the residence time required f o r complete combustion, i s reduced. Many e a r l y c o a l furnace designs emitted CO and smoke. Experience with coal f i r e d furnaces i s v a l i d here because the combustion of gases v o l i t i z e d from coal i s s i m i l a r to the combustion of gases v o l i t i z e d from hog f u e l . Smoke consists of hydrocarbons and i s emitted as a r e s u l t of incomplete combustion. I t was found that when a i r j e t s or steam j e t s were used i n the page 139 furnace combustion zone, that emissions were dramatically reduced and e f f i c i e n c y was increased. Niessen (18) c i t e s numerous examples of t h i s . The work of Mayer (18) i l l u s t r a t e s the e f f e c t s of o v e r f i r e a i r j e t s . Mayer sampled the gases at 45 locations i n a furnace f i r e d with bituminous co a l . His r e s u l t s are shown i n f i g u r e C.3.2. Mayer p l o t t e d l i n e s of equal heating value of the sampled gases. Figure C.3.2a has no o v e r f i r e a i r , f i g u r e C.3.2b has j e t s with a plenum pressure of 7 In.^O, and fi g u r e C.3.2c has j e t s with a plenum pressure of 10 i n . ^ O . In the case with no o v e r f i r e a i r , combustible gases are emitted from the furnace. In the case with strong o v e r f i r e a i r j e t s , no combustible gases leave the furnace as the l i n e of 0 heating value i s completely in s i d e the furnace e x i t . Thus, o v e r f i r e a i r j e t s reduce the residence time required f o r complete gaseous combustion by s t i r r i n g gases and adding oxygen. page 140 Figure C.3.2 Lines of Equal Heating Value i n a Furnace (18) Lamb et a l suggest that o v e r f i r e a i r j e t s can reduce gaseous emissions by increasing the excess a i r to high temperature regions, and by mixing the gases of quenched regions with hot gases. E f f e c t i v e macro scale mixing i s achieved, and the time required f o r micro scale mixing to occur i s reduced, reducing the residence time required f o r complete combustion. Mohr et a l (17) suggest that j e t s can co n t r o l the deposition of molten ash on furnace surfaces (slagging) by cooling regions i n the furnace that have a temperature higher than the ash fu s i o n temperature. O v e r f i r e a i r j e t s are also used i n hog f u e l furnaces to supplement the U.G.A. so that p a r t i c l e entrainment can be reduced. Ove r f i r e a i r j e t s are discussed further i n s e c t i o n C.5. Adams (19) proposes that vigorous flaming combustion i n the f u e l p i l e provides an i g n i t i o n source f o r gaseous combustion, preventing the gas flame from going out. Thus the s t a b i l i t y of page 141 gaseous combustion i s enhanced by combustion i n the f u e l p i l e . The following i s concluded about the combustion of gases: Combustion of gases i s l i m i t e d by mixing and not by the rate of chemical reaction. Combustion occurs almost instantaneously as combustible gases and oxygen are mixed. Macro mixing produces uniform time averaged p r o f i l e s of gas concentration and temperature i n the combustion zone. Uniform v e l o c i t y p r o f i l e s are produced downstream of the j e t s . The a i r to f u e l r a t i o may be l e s s than stoi c h i o m e t r i c Instantaneously, r e s u l t i n g i n the emission of unburnt f u e l gases. Intense small scale turbulence mixes gases on a micro scale, breaking up packets of non uniform gas concentration, thus reducing instantaneous deviations of gas concentration from the mean and reducing emissions. Ove r f i r e a i r j e t s reduce the residence time required f o r complete combustion by adding oxygen to the combustion zone, and by mixing the gases, thus reducing emissions. Combustion, therefore can be concentrated near the grate by the mixing e f f e c t of the o v e r f i r e a i r j e t s . Combustion i n the f u e l p i l e s t a b i l i z e s combustion of gases above the f u e l p i l e . C.4 Combustion of Entrained Fuel P a r t i c l e s I t i s t y p i c a l f o r ten to f i f t y percent of the hog f u e l fed page 142 int o a spreader stoker furnace to become entrained into the gas flow (16). Therefore, combustion of entrained p a r t i c l e s Is an important part of combustion i n the furnace. Adams (16) has compiled the r e s u l t s from three experimental studies and a computer simulation. They are b r i e f l y summarized i n t h i s section, along with other references. In one study by Adams, a sing l e f u e l p a r t i c l e was suspended by a f i n e wire attached to a micro balance and placed into a stream of a i r or N 2 i n a s p e c i a l l y constructed furnace. The gas temperature and v e l o c i t y were c o n t r o l l e d . The p a r t i c l e weight was measured as a function of time as i t dried, d e v o l i t i z e d and burned i n the furnace. I t was found that p a r t i c l e s do not change appreciably i n s i z e or shape as they dry and d e v o l i t i z e . The r e s u l t f o r a wet .47 inch thick Douglas f ur p a r t i c l e i s shown i n fi g u r e C.4.1. The curve can be divided Into three roughly l i n e a r sections; the f i r s t corresponds to drying, the second corresponds to d e v o l i t i z a t i o n and the t h i r d corresponds to char combustion. Simmons and Ragland (20) performed a s i m i l a r experiment with wood cubes with .4 inch and .8 inch sides and observed flaming during the drying and d e v o l i t i z a t i o n stages. Therefore the stages of drying, d e v o l i t i z a t i o n and char combustion probably overlap to some extent. page 143 Figure C.4.1 Combustion of a Hog Fuel P a r t i c l e (16) The v o l a t i l e matter content was estimated from the p a r t i c l e mass at the i n t e r s e c t i o n of the l i n e s approximating the d e v o l i t i z a t i o n and char burnout curves. I t was found to increase from about 85% to 90% (bone dry basis) as temperature increased from 600°F to 1400°F. Simmons and Ragland reported a v o l a t i l e content of about 90% (bone dry basis) estimated the same way. The v o l a t i l e content however, may be somewhat lower as char combustion may have occurred during the d e v o l i t i z a t i o n stage. The important r e s u l t s of Adam's experiment are that the v o l a t i l e content of hog f u e l i s 85% to 90% (bone dry basis) at the temperatures experienced i n a hog f u e l b o i l e r , and as p a r t i c l e s dry and d e v o l i t i z e , t h e i r s i z e does not change so that the density of d e v o l i t i z e d f u e l p a r t i c l e s i s only 10% to 15% of t h e i r o r i g i n a l , bone dry density. An experiment c i t e d by Adams (Malte) involved feeding p u l v e r i z e d bark into a c y l i n d r i c a l furnace that simulated page 144 conditions in s i d e a hog f u e l b o i l e r with one dimensional flow. I t was found that p a r t i c l e burnout was always completed i n le s s than one second f o r p a r t i c l e s with a mass mean s i z e of le s s than .08 inches. Thus p a r t i c l e s smaller than .08 inches w i l l burn completely i n a f u l l s i z e d hog f u e l furnace. An experiment was conducted by Pershing ( c i t e d by Adams) i n which entrainment and combustion of f u e l p a r t i c l e s were simulated i n a one dimensional gas flow. The apparatus i s shown schematically i n fig u r e C.4.2. A stream of f u e l p a r t i c l e s was i n j e c t e d i n t o a c y l i n d r i c a l furnace. A mixture of gases, preheated by a gas burner rose from the bottom of the furnace. About 3 fe e t below the point of entry was a p a r t i c l e c o l l e c t i o n system that r a p i d l y quenched the p a r t i c l e s f o r a n a l y s i s . P a r t i c l e s that were c o l l e c t e d there were assumed to correspond to p a r t i c l e s reaching the grate of a spreader stoker furnace. The apparatus d i d not model re-entrainment of p a r t i c l e s from the grate. About 6 fe e t above the point of entry was another s i m i l a r c o l l e c t i o n system. P a r t i c l e s c o l l e c t e d there corresponded to carryover. page 145 Figure C.4.2 Schematic of the Apparatus of Pershing (16) The p a r t i c l e s i z e was selected by a screen so that the p a r t i c l e s i z e was randomly d i s t r i b u t e d according to the screen mesh s i z e . The main determinant of the s i z e of wood and bark chips that go through the screen i s width (16). I t should be noted that the distance from the f u e l hopper to the grate i n a t y p i c a l hog f u e l spreader stoker furnace i s about 3 times the distance i n the apparatus, and the distance from the hopper to the e x i t of the furnace i s about 8 times as b i g . Pershing found that the r a t i o of entrained mass to i n i t i a l mass, and the r a t i o of carryover to i n i t i a l mass, decrease as p a r t i c l e s i z e increases. There i s no d i s t i n c t screen s i z e above which p a r t i c l e s do not become entrained. Carryover i s therefore unavoidable. Of the p a r t i c l e s entrained, the mass i s reduced by at l e a s t 90% (bone dry b a s i s ) . Chemical analysis of c o l l e c t e d p a r t i c l e s reveals that over a gas v e l o c i t y range of 6 to 20 page 146 feet/second, entrained matter i s completely d r i e d and d e v o l i t i z e d . A v e l o c i t y of 20 feet/second corresponds to a gas residence time of only .4 seconds. Drying and d e v o l i t i z a t i o n therefore, are very f a s t . Pershing found that p a r t i c l e s that f a l l onto the grate were, at the most, dried. Fuel p a r t i c l e s should therefore be able to f a l l to the grate, but, since the density of dry d e v o l i t i z e d p a r t i c l e s on the grate i s very low, the v e l o c i t y over the grate must be low i n order to prevent re-entrainment of p a r t i c l e s from the grate as discussed i n se c t i o n C.2. The mass of carbon i n the carryover divided by entrained mass increased from 0% to 1.5% f o r .16 inch to .24 inch wide p a r t i c l e s and from . 5% to 6% f o r 0 to .11 inch wide p a r t i c l e s as Pershing increased the gas v e l o c i t y from 6 to 20 feet/second. Thus, p a r t i c l e s t h i c k e r than .16 inch burn out much more thoroughly than p a r t i c l e s i n the range 0 to .11 inches thick. Also, Malte (16) demonstrated that f i n e l y p u l v e r i z e d p a r t i c l e s burn out completely i n hog f u e l furnace environments. Therefore, there i s a range'of p a r t i c l e s i z e s between the s i z e of p a r t i c l e s that Malte was experimenting with (mass mean s i z e l e s s than .08 inch) and about .24 inch thick, that become entrained r e a d i l y , and do not burn out thoroughly i n hog f u e l furnaces. Hog f u e l should therefore be put through a screen, so that p a r t i c l e s l e s s than .11 inch are separated out. The small p a r t i c l e s should be f i n e l y p u l v e r i z e d and i n j e c t e d into the furnace. Pershing also found that, as 0^ concentration was increased from 2% to 10%, at the e x i t of the experimental furnace, the mass page 147 r a t i o of unburned carbon to entrained matter was reduced from 6% to 2%. Thus entrained f u e l burns out more thoroughly as more oxygen i s provided. Also, as temperature increased from 1400°F to 1900°F entrained matter increased from 60% to 80% and the r a t i o of char carryover to entrained mass stayed constant. Entrainment therefore, increases with temperature, and the f r a c t i o n of unburned carbon i n the carryover i s unaffected by temperature. Adams (16) suggests that the rate of combustion of char i n a l l but the smallest f u e l p a r t i c l e s i s l i m i t e d by d i f f u s i o n of oxygen, and i s independent of the rate of chemical reaction. This i s consistent with Pershing's r e s u l t that the carbon content of carryover decreases as oxygen concentration increases, and i s independent of temperature. Combustion of carbon i n f i n e p a r t i c l e s however, i s dependent on the rate of chemical r e a c t i o n and therefore, temperature and i s not l i m i t e d by d i f f u s i o n of oxygen and oxygen concentration, as long as s u f f i c i e n t oxygen i s present f o r complete combustion. Under the conditions of Pershings experiment however, f i n e p a r t i c l e s burn out completely so that temperature does not a f f e c t the carryover carbon content. The computer simulation c i t e d by Adams (Malte) simulated p a r t i c l e s being i n j e c t e d h o r i z o n t a l l y into a uniform, v e r t i c a l gas flow f i e l d . P a r t i c l e drying and p y r o l y s i s are simulated but char combustion i s not. Malte found that large p a r t i c l e s have long residence times so that drying and d e v o l i t i z a t i o n are completed low i n the furnace. Small p a r t i c l e s have short residence times but, because t h e i r surface area to volume r a t i o i s large, they dry page 148 and d e v o l i t i z e quickly and combustion i s completed low i n the furnace. Intermediate s i z e d p a r t i c l e s f i n i s h d e v o l l t i z i n g high i n the furnace which suggests that they would r e s u l t i n the most carryover. The r e s u l t s of t h i s study suggest that intermediate s i z e d p a r t i c l e s should be reduced i n si z e before in t r o d u c t i o n to a furnace, as was previously concluded. Tillman (14) has c i t e d r e s u l t s of experiments conducted on a f u l l s i z e d spreader stoker furnace by Junge, at Oregon State U n i v e r s i t y . Junge found that carryover of large p a r t i c l e s decreases as excess a i r i s increased. Above 75% excess a i r however, carryover increases. This r e s u l t corresponds to the r e s u l t of Pershing, that f u e l p a r t i c l e burnout i s more complete i f s u f f i c i e n t oxygen i s present. As excess a i r gets too high however, the increase i n f u e l entrainment, and decreased residence times of p a r t i c l e s cause carryover to increase. Carryover of f i n e p a r t i c l e s increases with excess a i r . Excess a i r lowers the gas temperatures,and increases v e l o c i t y . Low temperatures r e t a r d the burn out of carbon i n small p a r t i c l e s , and high v e l o c i t i e s increase entrainment and decrease p a r t i c l e residence times. Thus the rate of char combustion i n large p a r t i c l e s increases as the 0^ concentration of the surrounding gases increases while i t does not f o r small p a r t i c l e s . Tillman found that as the r a t i o of O.F.A. to U.G.A. increases, carryover decreases and completeness of combustion of the carryover increases. This i s because, as the f r a c t i o n of o v e r f i r e a i r to undergrate a i r i s increased, U.G.A. i s decreased so that v e l o c i t i e s above the grate decrease and entrainment from page 149 the grate i s therefore reduced. As the b o i l e r steam load increases, gas v e l o c i t i e s and the rate of f u e l feed increase, so that emissions of carryover increase with b o i l e r load. Spurrel (21) measured the carryover rate and load i n a hog f u e l b o i l e r and found that the maximum load had to be l i m i t e d to the point where the allowable emissions of p a r t i c l e s was exceeded. The e f f e c t s of v a r i a b l e s a f f e c t i n g combustion of entrained f u e l p a r t i c l e s are summarized as follows: Fuel p a r t i c l e s do not change i n s i z e or shape as they dry and d e v o l i t i z e . The mass of f u e l p a r t i c l e s i s reduced by 85% to 90% (dry basis) as they dry and d e v o l i t i z e . Drying and d e v o l i t i z a t i o n occur i n le s s than .4 seconds under furnace conditions. P a r t i c l e s that f a l l onto the grate are, at the most dried, so that they can enter the furnace above the o v e r f i r e a i r nozzles and f a l l to the grate. P a r t i c l e s that reach the grate however, lose up to 90% of t h e i r dry mass as they d e v o l i t i z e , so that v e l o c i t i e s above the grate must be kept low to prevent entrainment from the grate. P a r t i c l e s i n the s i z e range of 0 to .11 inch t h i c k do not burn out completely. Very small p a r t i c l e s and p a r t i c l e s l a r g e r than .11 inch t h i c k burn out more completely. Therefore, p a r t i c l e s less than .11 inches t h i c k should be page 150 screened out and pulverized before being i n j e c t e d into a furnace. Burnout of a l l but the smallest p a r t i c l e s increases with oxygen concentration, and i s not a f f e c t e d by temperature. Carryover of these p a r t i c l e s decreases as excess a i r increases u n t i l a c r i t i c a l value of about 75% excess a i r i s reached. Above t h i s point, decreased residence time and increased entrainment cause an increase i n carryover. Burnout of f i n e f u e l p a r t i c l e s does not increase with oxygen concentration. Thus, carryover of f i n e p a r t i c l e s increases with excess a i r . Carryover decreases as the r a t i o of o v e r f i r e a i r to under grate a i r increases because entrainment from the grate i s reduced as U.G.A. i s reduced. Carryover increases with b o i l e r load. Entrainment, combustion and carryover of f u e l p a r t i c l e s are dependent on the p a r t i c l e s i z e d i s t r i b u t i o n . The e f f e c t s of p a r t i c l e s i z e on entrainment, combustion and carryover, and r e l a t e d v a r i a b l e s are summarized i n table C.4.1. Research i s required i n the e f f e c t s of a three dimensional flow f i e l d , i n c l u d i n g o v e r f i r e a i r j e t s on f u e l p a r t i c l e d i spersion, residence time and entrainment. page 151 Table C.4.1 The E f f e c t of P a r t i c l e Size on Entrainment, Combustion and Carryover Size Range very small < .11 inch > . l l inch completeness of burnout Burn out quickly due to high r a t i o of surface area volume P a r t i c l e s are e a s i l y entrained and r a t i o of surface area volume i s not high. Less complete combustion. Long residence time r e s u l t s i n complete burnout of entrained p a r t i c l e s . excess a i r None, as long as s u f f i c i e n t f o r combustion. Rate of combustion increases with 0^ concentration, and therefore excess a i r . At high excess a i r , increased gas v e l o c i t i e s lead to increased entrainment, decreased residence time and increased carryover. temperature Combustion rate increases with temperature since l i m i t e d by r e a c t i o n rate. No e f f e c t since combustion i s l i m i t e d by d i f f u s i o n . r a t i o of O.F.A. U.G.A. L i t t l e , s i n c e p a r t i c l e s are entrained. Reduced entrainment of f u e l p a r t i c l e s and re-entrainment of dr i e d devolatized p a r t i c l e s from the grate r e s u l t s i n l e s s carryover and more complete combustion as ^ p ^ increased. b o i l e r load Gas flow rates and gas v e l o c i t i e s increase as load increases. Residence time decreases, entrainment increases and therefore, emission of carryover increases with load. Completeness of combustion of carryover i s reduced. C.5 Furnace Aerodynamics Combustion i n the f u e l p i l e , gaseous combustion and combustion of entrained f u e l p a r t i c l e s are a l l coupled with the aerodynamics of the gas flow f i e l d i n the furnace. The main elements of the gas flow f i e l d are o v e r f i r e a i r page 152 j e t s , r e c i r c u l a t i o n zones and channeling of the flow. Buoyancy, caused by an uneven temperature d i s t r i b u t i o n above the grate, may a f f e c t channeling of the flow f i e l d . Discussed i n t h i s section are o v e r f i r e a i r j e t models, r e c i r c u l a t i o n zones and channeling of the flow and buoyancy e f f e c t s . C.5.1 Overfire A i r Jet Models Over f i r e a i r j e t s are used to mix furnace gases, and to provide combustion a i r so that under grate a i r flow and therefore entrainment of p a r t i c l e s from the grate can be minimized. Four types of i d e a l i z e d models can be found i n l i t e r a t u r e . They are: 1 A j e t entering into an unconfined, quiescent flow f i e l d with density at the nozzle, d i f f e r e n t than the density f a r away from the nozzle; the e f f e c t s of buoyancy and combustion are ignored. 2 Same as 1 except that the j e t i s def l e c t e d up or down by buoyancy e f f e c t s . 3 Same as 1 except that the flow f i e l d has a uniform non zero crossflow v e l o c i t y upstream of the j e t nozzle. 4 Same as 1 except that combustion i s included i n the model; the temperature d i s t r i b u t i o n i s modeled. 5 Same as 3 except that combustion i s included i n the model; the p o s i t i o n of the point of maximum temperature and the corresponding d e f l e c t i o n of the j e t are modeled. page 153 Cases 4 and 5 are not considered here. Models can be found i n Niessen (18). Mohr et a l (17) conducted an extensive l i t e r a t u r e review i n 1975 i n c l u d i n g the modeling of o v e r f i r e a i r j e t s . Many models for j e t s from i t are presented by Niessen (18). Some of them are presented i n t h i s section. A l l of the j e t models presented are v a l i d only f o r Reynolds Number: R e « d p V o > 2 0 0 0 C.5.1.1 v do-hydraulic diameter of the nozzle Vo—mean v e l o c i t y at the e x i t of the nozzle ^-kinematic v i s c o s i t y of f l u i d at the nozzle e x i t In hog f u e l furnaces, Re i s u s u a l l y i n the order of 10 , so that Re need not be considered. Ov e r f i r e a i r i s introduced into a hog f u e l b o i l e r at a temperature between 70°F and 600°F. The gases i n a hog f u e l b o i l e r have a temperature of about 2000°F. Therefore, the density of the over f i r e a i r i s t y p i c a l l y three times the density of the furnace gases. The density d i f f e r e n c e causes buoyant forces to d e f l e c t the j ets downwards. Mohr et a l derived the following expression to r e l a t e the r a t i o of buoyant force to drag force near the nozzle of a j e t i n crossflow with as shown i n fi g u r e C.5.1.1: P- - . 3 3 x ^ C-^-l) C.5.1.2 F d v 8 2 L ' g J page 154 Fd=drag force per un i t length of the j e t caused by the crossflow Fb= buoyant force per u n i t length of the j e t Vg=crossflow v e l o c i t y P Q=density of gas at nozzle e x i t p -density of gas an i n f i n i t e distance from the nozzle e t I ! I f t I t V S /Y Fb Fd Figure C.5.1.1 Parameters Near The Nozzle of a Buoyant J e t i n Crossflow Experimental r e s u l t s by Mohr et a l (17) in d i c a t e that f o r < 1, buoyancy e f f e c t s can be ignored. T y p i c a l values of the parameters of equation C.4.1, i n a hog f u e l furnace, are: do=5 Po inches, Vg=15 feet/second, — •= 3. Therefore i n a hog f u e l furnace, t y p i c a l l y : page 155 |£ ° . 3 3 x ^ - z x 12 [ 3-1] = .04 « 1 15 2 Thus the e f f e c t s of buoyancy on j e t s can be Ignored i n most hog f u e l furnaces. Case 1 Jets i n Quiescent Flow; no Buoyancy In these models, the e f f e c t s of combustion i n the j e t , buoyancy and crossflow are neglected. Figure C.5.1.2 i l l u s t r a t e s a j e t i n quiescent f l u i d . Figure C.5.1.2 Jet i n Quiescent flow, No Buoyancy Niessen (18) l i s t s the following expression f o r the a x i a l decay of c e n t r e l i n e v e l o c i t y and concentration i n j e t s with round nozzles: Vm p 1/2 do ,-. =6 . 3x f — ] x C. 5.1. 3 Vo ^ P  J (^ +.6do) 8 I f the concentration of a species i n the f l u i d i n the j e t i s d i f f e r e n t than the concentration at x=<o, the a x i a l decay of concentration can be modeled as (18): page 156 Cm p 1/2 do 5.0xf -°] x . C.5.1.4 - ^ p •> (x +.6do) CO g X These expressions are v a l i d f o r -> 8. do x= a x i a l distance from the nozzle Vm= mean c e n t r e l i n e v e l o c i t y at x Vo-mean c e n t r e l i n e v e l o c i t y at the nozzle e x i t Cm-mean c e n t r e l i n e concentration at x Co- mean c e n t r e l i n e concentration at the nozzle e x i t Niessen (18) l i s t s the following expressions f o r r a d i a l v e l o c i t y and concentration d i s t r i b u t i o n s f o r j e t s with round nozzles: ^=ex P(-96(§) 2] C.5.1.5 Vo ^ -exp[ -57.5 ( I ) 2) C.5.1.6 Co x These expressions are v a l i d f o r ^ q > 8. r=radial distance from the center l i n e V- mean v e l o c i t y , a distance r from the center l i n e C—mean concentration, a distance r from the center l i n e The spread of a j e t , ©, i s defined as the h a l f angle at which V =1/2 Vm, or C-l/2 Cm. This angle i s independent of the distance from the nozzle i n the f u l l y developed region 8 nozzle diameters page 157 downstream. In round j e t s , f o r v e l o c i t y p r o f i l e s , © —4.85° and fo r concentration p r o f i l e s , 0 = 6 . 2 ° . c The mass flow rate through the cross section of a round isothermal j e t i n no crossflow can be estimated by (18): I x p 1/2 — _..32{ _!) x g j C.5.1.7 mo y- p •> ^doJ 8 This expression i s v a l i d f o r Re > 25000, %> 6. do mx—mass flow rate at x mo—mass flow rate at nozzle The flow rate of mass entrained into the j e t , from the nozzle to x, equals mx-mo. The mass flow rate and entrained mass flow p 0 x increase l i n e a r l y with x. I f — -3, and ^ 0""6, mx—3.3 mo. Thus, s j u s t a few feet from the nozzle, the mass entrained by the j e t i s twice as much as the o v e r f i r e a i r flow rate. Niessen (18) l i s t s the following expressions f o r a 2 dimensional j e t i s s u i n g from a s l o t of width y Q: Vm -IIZp 1/2 -2.48 f - +.61 x (—) C.5.1.8 Vo v y J p Cm -1/2 p h -2.00f - +.6] x ( — ) C.5.1.9 co v0 v ^-exp ( - 7 5 ( ^ ) 2 ) C.5.1.10 ^ „ . 5 x ( l + c o s [ - ^ ] ) C.5.1.11 L -exp[-36.6(^) 2) C.5.1.12 page 158 ^ -.5x(l +cos[ A | ] ) C.5.1.13 mx p 1/2 1/2 ~ =.508f - 1 x ^ 1 C.5.1.14 mo p y mx and mo are mass flow rate into the j e t per u n i t length of the s l o t . Designers of furnaces often need to know how f a r an o v e r f i r e a i r j e t penetrates e f f e c t i v e l y into the combustion zone of a furnace. For a j e t i n no cross flow, the throw, Th, i s defined as the a x i a l distance from the nozzle of the j e t at which the ce n t e r l i n e mean v e l o c i t y has decayed to a s p e c i f i e d terminal v e l o c i t y . Kaiser and McCaffery (22) recommend a terminal v e l o c i t y of 25 feet/second. They present formulae to c a l c u l a t e the throw, but they do not appear to take into account the differ e n c e i n density between the j e t and the furnace gas. However, i f Vm-25 feet/second i s substituted into equation C.5.1.3, and the equation i s rearranged: T W o ( 6.3| — 1 xvo-,6 ) C.5.1.15 Th= throw of a round j e t i n no crossflow (feet) do-nozzle diameter (feet) Vo=nozzle c e n t r e l i n e v e l o c i t y ( f t . / s . ) For a s l o t j e t , equation C.5.1.8 i s s i m i l a r l y rearranged with VB=25 feet/second: page 159 Th=[ 9.84xlO" 3Vo 2x( — ) -.6)x y C.5.1.16 y o=width of the s l o t (feet) I f po/p = 3, and Vo -150 feet/second, f o r a round j e t with or f o r a s l o t j e t , with yQ"=5 inches: Th-( 9.84xl0" 3xl50 2x( 3) -.6]x (5/12) -267.5 feet V e l o c i t y decays much more slowly i n s l o t j e t s , than i n round j e t s . Case 2 Jets i n Quiescent Flow; No Buoyancy Since the e f f e c t of cross flow i s much greater than the e f f e c t of buoyancy i n hog f u e l furnaces, there i s no point i n attempting to model buoyancy without crossflow. Therefore no models f o r j e t s i n quiescent flow with buoyancy are presented here. Some expressions f o r the t r a j e c t o r y of buoyant j e t s i n quiescent flow appear i n Niessen (18). Case 3 Jets i n Unconfined, Uniform Crossflow; no Buoyancy Cross flow has two e f f e c t s on j e t s . The c e n t r e l i n e of the j e t i s d e f l e c t e d i n the d i r e c t i o n of the cross flow and the cross s e c t i o n of the j e t becomes d i s t o r t e d . The j e t cross s e c t i o n becomes kidney shaped, and r e c i r c u l a t i o n occurs w i t h i n i t . Measurements show p e r i o d i c vortex shedding by the j e t and decreased pressure downstream of i t (18). Case 1 models should 8 do=5 inches: 1/2 Th=(5/12)( 6.3 3 x 150-.6 ) -27 f e e t page 160 therefore, be used with caution f o r j e t s i n cross flow. A j e t i n cross flow i s i l l u s t r a t e d i n figure C.5.1.3. Figure C.5.1.3 Jet i n Uniform Crossflow Case 3 models do not model buoyancy, combustion or confinement by furnace walls or opposing j e t s , or non uniformity of the cross flow v e l o c i t y . The c e n t r e l i n e of a j e t i n cross flow i s defined as the locus of maximum v e l o c i t y . Niessen (18) c i t e s several models f or the t r a j e c t o r y of the centerlines of round j e t s i n crossflow. The parameter: P v 2 M" 8 8 C.5.1.17 >o vo 2 i s included i n the models. Vo=mean cen t r e l i n e v e l o c i t y at the nozzle e x i t page 161 Vg—mean cross flow v e l o c i t y The f ollowing expressions are recommended by Niessen (18) because they are v a l i d f o r ranges of M that are encountered i n furnaces: ?< u l . 12 ' X 2. 64 „ r •• i r, 5f - M X ( - ) C.5.1.18 do do i s v a l i d f o r 0 < M < .023, ao -0 y~ the v e r t i c a l d e f l e c t i o n of the c e n t r e l i n e at x ao= the angle between the nozzle axis and the X axis H o - " 1 " * 4 ) 3 + Ho t a n ( a o ) c - 5 - x - 1 9 i s v a l i d f o r .001 < M < .8, and can be used f o r ao* 0. Ivanov ( c i t e d from Niessen (18)) measured j e t t r a j e c t o r i e s f o r h o r i z o n t a l rows of round j e t s and found that c l o s e l y spaced j e t s are d e f l e c t e d more than broadly spaced ones. Unfortunately, a general c o r r e l a t i o n f o r the d e f l e c t i o n of rows of j e t s as a function of v e l o c i t y and density was not found i n Niessen's book. No expressions were found f o r the v e l o c i t y d i s t r i b u t i o n i n j e t s i n cross flow, since i t i s d i f f i c u l t to measure. Therefore, the throw of a j e t i n crossflow can not be predicted. However, j e t penetration, Lj i s defined as the h o r i z o n t a l distance from the nozzle at which the c e n t r e l i n e of the j e t becomes n e g l i g i b l y close to being aligned i n the d i r e c t i o n of the cross flow v e l o c i t y . The following expression, for a j e t i s s u i n g from a round nozzle, i s page 162 from Niessen (18): L i do C.5.1.20 C.5.2 R e c i r c u l a t i o n Zones and Channeling of the Flow In a hog f u e l furnace, the flow f i e l d i s much more complicated than j e t s i n uniform, unconfined cross flow. The accuracy of the models presented i n se c t i o n C.5.1 may be grossly a f f e c t e d by the i n t e r a c t i o n of j e t s with opposing j e t s , furnace walls, combustion and a non-uniform v e l o c i t y p r o f i l e above the f u e l grate. Adams (11) observed the gas flow i n hog f u e l and Kr a f t recovery b o i l e r s and found that flow patterns s i m i l a r to f i g u r e C.5.2.1 occur. In recovery and hog f u e l b o i l e r s , each side wall u s u a l l y has a bank of opposing nozzles. The j e t s c o l l i d e i n the middle of the furnace and a column of gases r i s e s there. Overfire a i r and furnace gases are mixed gradually, and mixing i s not completed near the grate. Large r e c i r c u l a t i o n zones occur at the walls. The r e c i r c u l a t i o n zones were estimated to take up about 77% of the furnace cross section. Adams concluded that j e t penetration i n furnaces with opposing j e t s i s not adequately represented by j e t s i n uniform, unconfined crossflow. The column of f a s t moving gas i n the middle of the furnace i s a channeling of the flow. In the region of high v e r t i c a l v e l o c i t y i n the middle of the furnace, p a r t i c l e s are more l i k e l y to be entrained, than they would be i n a furnace with a uniform v e l o c i t y p r o f i l e and the same f l u e gas flow rate. Since there i s not vigorous mixing, gases and page 163 p a r t i c l e s that are i n the region of channeled flow have no opportunity to be transfered into other areas of the furnace cross s e c t i o n . Residence times i n the channeled region are short. Since there i s l i t t l e mixing, gas concentration and temperature are probably not uniform across the furnace. Since the gas concentrations are not uniform, the a i r to f u e l r a t i o i s not uniform throughout the furnace cross s e c t i o n . In order to prevent emissions, the combustion a i r flow rate must be high enough to allow complete combustion to occur throughout the furnace, even where the furnace gases do not mix thoroughly with the o v e r f i r e a i r . Thus, e l i m i n a t i o n of channeling would r e s u l t i n a greater uniformity of gas concentration i n the furnace, and the flow rate of combustion a i r , and therefore v e l o c i t i e s i n the furnace, could be reduced. Figure C.5.2.1 R e c i r c u l a t i o n Zones and Channeling i n a Recovery B o i l e r (11) page 164 C.5.3 Buoyancy E f f e c t s Buoyancy e f f e c t s occur when gas temperature gradients are present along h o r i z o n t a l planes i n the flow. A common cause of t h i s i s tramp a i r , which i s a i r at the ambient, outside temperature that leaks into the furnace. Temperature gradients can also occur i f combustion i s not uniform across the f u e l grate, or i f combustion a i r bypasses the f u e l p i l e through bare areas on the f u e l grate. Lamb et a l (1) assumed that the flow could be modeled as two p a r a l l e l v e r t i c a l , unmixing and f r i c t i o n l e s s streams of hot and c o l d gas as shown i n f i g u r e C.5.3.1. They assumed that cold gas does not accelerate but the hot gas does. The flows are modeled by the B e r n o l l i equation and pressure i s assumed to be a function of the v e r t i c a l ordinate, only. Figure C.5.3.1 Buoyancy i n a Furnace hot gas stream: Vh 2=Vh 2+ C.5.3.1 page 165 AP -pressure drop over increment of height bty Vh(y.) - v e r t i c a l v e l o c i t y i n hot stream Vho= v e r t i c a l v e l o c i t y i n the hot stream at A£=0 Ph= density i n the hot stream c o l d gas stream: AP 0 - — -2gA£ C.5.3.2 c p = density i n the c o l d stream c or rearrange C.5.3.2 AP - p gA£ C.5.3.2a C s u b s t i t u t e C.5.3.2a into C.5.3.1 to get: P Vh 2- Vh2+ 2gA*(— -1) C.5.2.3 p " Tc c Th and Tc are the temperatures of the hot and c o l d gas streams Th Vh 2- Vh2+ 2gA£(_ -1) C.5.3.4 0 Tc V e l o c i t i e s and temperatures were measured above the f u e l bed of a refuse i n c i n e r a t o r . Equation C.5.3.4 was found to f i t the data w e l l when Tc was set equal to the ambient, outside temperature of 70°F. I t was concluded that i n poorly sealed furnaces, tramp a i r i s the main cause of buoyancy i n the flow. page 166 Since the product of the h o r i z o n t a l cross s e c t i o n a l are of the hot stream times Vh must be constant to s a t i s f y c ontinuity, as Vh increases the hot gas zone gets narrow. Thus buoyancy e f f e c t s , as we l l as the r e c i r c u l a t i o n zones caused by o v e r f i r e a i r j e t s , can cause channeling of the flow and uneven v e l o c i t y p r o f i l e s across the furnace cross section. Buoyancy e f f e c t s can occur i f regions of v e r t i c a l s t r a t i f i c a t i o n of gases, or channeling of the flow occur In the model. These can be i d e n t i f i e d with r e s u l t s of smoke flow v i s u a l i z a t i o n and v e l o c i t y measurements i n the model. C.5.4 Conclusions about Aerodynamics i n Furnace Flow F i e l d s Bouyancy e f f e c t s can be ignored, when modelling o v e r f i r e a i r j e t s i n hog f u e l b o i l e r s . Mathematical models f o r unconfined j e t s may not be accurate when applied to opposing banks of j e t s i n hog f u e l furnaces. Overfire a i r j e t s i n have been observed to cause large r e c i r c u l a t i o n zones to occur near the walls, and a region of high v e r t i c a l v e l o c i t y to occur near the middle of recovery b o i l e r s . Channeling of the flow probably r e s u l t s i n increased carryover and gaseous emissions i f intense mixing does not occur. Buoyancy e f f e c t s , r e s u l t i n g i n channeling of the flow, can occur i f there are h o r i z o n t a l temperature gradients i n the flow. Horizontal temperature gradients are caused by tramp a i r , uneven combustion on the f u e l grate, or channeling i n the flow. C.5.5 Research Needs In Aerodynamics of Furnace Flow F i e l d s The a p p l i c a b i l i t y of the models presented i n se c t i o n C . l should be tested by comparing predictions of the models with the page 167 r e s u l t s of the simulation of the gas flow f i e l d of a hog f u e l b o i l e r . The e f f e c t of channeling of the flow and r e c i r c u l a t i o n zones on p a r t i c l e entrainment and residence time are not well understood. Measurements of the residence time of gas and p a r t i c l e s i n a furnace model, at various r a t i o s of o v e r f i r e a i r to undergrate a i r flow rates would provide u s e f u l information f o r the design and operation of hog f u e l b o i l e r s . Numerical or experimental models demonstrating the e f f e c t s of combustion on the gas flow f i e l d would provide knowledge u s e f u l f o r the design of furnaces. A model c o n s i s t i n g of opposing o v e r f i r e a i r j e t s , and a uniform cross flow should be constructed, so that c o r r e l a t i o n s f o r j e t v e l o c i t y p r o f i l e s and t r a j e c t o r i e s as well as r e c i r c u l a t i o n zones, can be produced f o r various r a t i o s of O.F.A. to under grate a i r , nozzle s i z e s , and spacing. The c o r r e l a t i o n s would provide models f o r furnace design that would be more r e a l i s t i c than the models presented i n se c t i o n C.5.1. page 168 Appendix D Properties of Hog Fuel Hog f u e l c onsists of p a r t i c l e s of wood and bark. I t i s c a l l e d "hog f u e l " because i t i s broken into pieces by a machine c a l l e d a hammer hog, i n preparation f o r f i r i n g . In table D.l, t y p i c a l materials that constitute hog f u e l , and t h e i r s i z e and moisture content, are l i s t e d . Moisture content i s c a l c u l a t e d on a wet bas i s . I t i s c a l c u l a t e d as the mass of moisture i n a sample of f u e l divided by the mass of the wet f u e l , expressed as a percent. Table D.l Size Ranges and Moisture Contents of Ty p i c a l Components of Hog Fuel (23) Component Size range Moisture content inches percent Bark .031-4 25-75 Coarse Wood Residues .031-4 30-60 Planer Shavings .031-.5 16-40 Sawdust 10" 7-.031 25-40 Sanderdust 10" 7-.25 2-8 The prevalent species of trees, used f o r hog f u e l i n the P a c i f i c North West are Douglas f i r , true f i r s , alder, ponderosa pine, western hemlock, spruces, l a r c h , cedars and redwood (23). Wood and bark c o n s i s t of combustible organic material, water and inorganic, incombustible ash. The organic material consists of c e l l u l o s e , hemicellulose and l i g n i n . The organic material consists of the elements hydrogen, carbon, oxygen, a very small amount of nitrogen and traces of other elements. The chemical composition of hog f u e l can be expressed as a proximate a n a l y s i s , or an ultimate a n a l y s i s . Proximate analysis of bark and sawdust are l i s t e d i n table D.2. In a proximate an a l y s i s , the f u e l i s heated to a temperature of at l e a s t 440°F, at which some of the organic matter vaporizes, and the r e s t of the organic page 169 matter i s converted to char, which Is almost pure carbon. The organic matter that vaporizes i s c a l l e d " v o l a t i l e matter". Proximate and ultimate analysis are expressed as a bone dry basis, i n which the constituent mass i s divided by the mass of dry f u e l , and expressed as a percent. Table D.2 Ty p i c a l Proximate Analysis of Moisture-Free Wood Fuels (23) Species V o l a t i l e Matter Char Ash percent percent percent BARK Hemlock 74 3 24 0 1.7 Douglas F i r 70 6 27 2 2.2 ( o l d growth) Douglas F i r 73 0 25 8 1.2 (young growth) Grand F i r 74 9 22 6 2.5 White F i r 73 4 24 0 2.6 SAWDUST Hemlock 84 8 15 0 0.2 Douglas F i r 86 2 13 7 0.1 White F i r 84 4 15 1 0.5 Ponderosa Pine 87 0 12 8 0.2 Ultimate analysis of hogged f u e l bark are l i s t e d i n table D.3. An ultimate analysis consists of the mass f r a c t i o n s of the elements c o n s t i t u t i n g the organic matter, and the ash. The ash content of hog f u e l i s often higher than the ash content of wood or bark because sand and s a l t often get into the f u e l during t r a n s p o r t a t i o n and handling. page 170 Table D.3 T y p i c a l Ultimate Analysis f o r Moisture-Free Samples of Hog Fuel Bark (23) Component Douglas F i r Western Hemlock Average of 22 samples percent percent percent Hydrogen 6.2 5.8 6.1 Carbon 53.0 51.2 51.6 Oxygen 39.3 39.2 41.6 Nitrogen 0.0 0.1 0.1 Ash 1.5 3.7 0.6 Higher heating values of wood and bark are l i s t e d i n table D.4. The heating value i s defined as the energy released per u n i t mass of dry f u e l , when pre-dried f u e l i s burned at a s p e c i f i e d temperature, under stoichiometric, adiabatic conditions. Table D.4 T y p i c a l Heating Values f o r Moisture-Free Bark and Wood (23) Species Heating Value BTU/lb wood bark Heating Value f t - l b / s l u g x l O 6 wood bark Douglas F i r 9,200 10,100 231,000 274,000 Western Hemlock 8,500 9,800 213,000 246,000 Ponderosa Pine 9,100 229,000 Western Redcedar 9,700 8,700 243,000 218,000 Western Alder 8,000 8,410 200,000 211,000 page 171 Appendix E In t h i s section, the r e s u l t s of the p.w.a. experiments are presented. Variables are defined i n sec t i o n 7. Some figures have been reproduced i n set i o n 8. E . l . l Case 1 V e l o c i t y i n the Plane Y=2.8 f t page 173 E. 1.2 Case 1 V e l o c i t y i n the Plane Y«=8.1 f t E.1.3 Case 1 V e l o c i t y i n the Plane Y=14.5 f t E.1.4 Case 1 V e r t i c a l V e l o c i t y , W i n the Plane Z=-4.5 f t i n \1* E.1.8 Case 1 V e r t i c a l V e l o c i t y , W i n the Plane Z-1.6 f t >a&e Me -o\ai A -i t ©a& e 182 21 Cas e t a l c i t y * \3 .6 E.1.12 Cas e 2 V e l o c i t y i n the Plane Y=8.1 f t vP 2_ 2 , MeA o C A -C&se i t \%6 2' 3-C & s e page 187 E.1.17 Case 3 V e l o c i t y i n the Plane Y-14.5 f t in N * . LD f\J-00 — i 2 -O or t r La or O y.\v ^ Case /. 4 Vert* E.1.19 Case 4 Horizontal V e l o c i t y , U i n the Plane Z=-4.5 f t page 191 50 > FT K ~ —x— "A ? • 6 ~&—^ S 2«!— -—AT ^ -50 100 Z=1.65 FT w r-X A—A-L_ . — A -50--50 100 Z=.065 { > -x-x-p 50--50 too O 0 o < pa-** ; Z=-4.4I 9 FT Legend J i u x w ^ / <-x-x- A— -—X-- —iX—., "X » 1 8 10 12 14 16 18 Y, FT. E.2.1 Case 1 V e l o c i t y P r o f i l e s i n the Plane X--.25 f t , Y as Abcissa page 192 6 0 4 0 -Z= 10.98 5 F T * — x A /V .—A , * ^ - x A A-— X — A— .. A "1 •-• H 1 "1 T" "I " I " 1 1 ' 100 -100 100 Z=1.6^ F - x - x \ * - x -v/ •X-X—X^^ Z= .065 FT - x - x - * " * X~-><-^ -'X^ r> 1 - 1 0 0 -- 2 0 0 . 100 in C> 0 < w -100 Z = - 4 . 4 5 9 F T 2 * S — A u x w - A ; ;—^ ? - I "y— 1 \ * - * ~ x - — . 1 1 — 1 1 a 1 Y, FT. E.2.2 Case 1 V e l o c i t y P r o f i l e s i n the Plane X=1.6 f t , Y as Abcissa page 193 6 0 100 Z=10.985 F T A - ^ A - A : ii A — Q - ^ A AH "—-ft— —*-= x - x - * - X — X N Z=1.65 F T - x - x - x - x - x - k -X—^ ~A A t - 1 0 0 100 x - x - * x - x ^ Z= .065 F T t - X X T ' * ; - 1 0 0 - 2 0 0 100 in 8 0 to < -100 x - * i * - x Z = - 4 . 4 5 9 F T L e 9 o n d x w c TVS / -^ ^. ~ ^ X — 1 1 1 Y, FT. E.2.3 Case 1 V e l o c i t y P r o f i l e s i n the Plane X=3.5 f t , Y as Abcissa page 194 E.2.4 Case 1 V e l o c i t y P r o f i l e s i n the Plane X=5.4 f t , Y as Abcissa page 195 E.2.5 Case 1 V e l o c i t y P r o f i l e s i n the Plane X-7.3 f t , Y as Abcissa page 196 E.2.6 Case 2 V e l o c i t y P r o f i l e s i n the Plane Z=1.6 f t , Y as Abcissa (continued on next page) ^—L < \ "T Legend ^ u x w • < ^  ~~& ' & ' A — 0 2 4 6 8 10 12 14 16 18 Y , FT E.2.6 Case 2 V e l o c i t y P r o f i l e s i n the Plane Z-1.6 f t , Y as Abcissa (continued from l a s t page) page 198 E.2.7 Case 4 V e l o c i t y P r o f i l e s i n the Planes Z — 4 . 5 f t and Z=1.6 f t , Y as Abcissa page 199 E.3 Cases 1, 2, 3 and 4 V e r t i c a l Component of V e l o c i t y , W, X as Abcissa T 1 > S> r , > < < ( > • L 1 L • • • • • • • • • • 1 r « 1 f 9 1 page 200 I N T E N S I T Y , F T / 5 • - : > 1 0 0 o - : > 8 0 , < 1 0 0 - - : > 6 0 , < 8 0 : > 4 0 , < 6 0 : > 2 0 , < 4 0 :>0, < 2 0 o D 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N R C E C E N T E R L I N E E.4.1 Case 1 Turbulence Intensity, v' i n the Plane Y-2.5 f t o o o o o C D cn o L J 00 z ° L J rx C J o O in' o > ^ a ^ DO ° CE to H« O L J L J C O o C D I O C M I i i o in. i r 1 < > < • > 0 i 0 i c r , s < • < 1 i i i ' 1 I , • 1 : ! < 1 1 , c 1 < 1 J : i i , ! ! i C 1 I ! • i i ;" i i i • • • ' 1 page 201 I N T E N S I T Y , F T / 5 B - : > 1 0 0 B - : > 8 0 , < 1 0 0 B = : > 6 0 , < 8 0 O -= : > 4 0 , < 6 0 D «= : > 2 0 , < 4 0 • - :>0, < 2 0 •1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E E.4.2 Case 1 Turbulence Intensity, v' i n the Plane Y-8.1 f t r ~ ~ ~ 1 > » < ; 0 < < r < i < i < 1 • 1 i — 1 1 I' 1 1 i < i • i >- < i I J 1 1 : ! I > < * "5 > < \ 1 1 1 i r — » > > t > j i i > > i r s > 1 r - « r ^ i — -< 1 page 202 I N T E N S I T Y , F T / S a - : > 1 0 0 : > 8 0 , < 1 0 0 : > 6 0 , < 8 0 : > 4 0 , < 6 0 : > 2 0 , < 4 0 : > 0 , < 2 0 D -B •= O •= D «= •1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N R C E C E N T E R L I N E E.4.3 Case 1 Turbulence Intensity, \ i n the Plane Y-14.5 f t page 203 o r \ o CD O in o •9-•—1 CD t o U. CE o L_ o LJ 2 CD CE o ZD CO o oo O D e-4 LJ CD LJ L_ o •> >- „ o to o o i I l T 1 1 1 I 1 I I I i . O 0.0 l.O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , FEET FROM FURNRCE CENTER LINE INTENSITY, F T / S a - :>lDO : > 8 0 , < 1 0 0 : > 6 0 , < 8 0 : > 4 0 , < 6 0 : > 2 0 , < 4 0 :>0, < 2 0 B -B = O = D = E.4.4 Case 1 Turbulence Intensity, v' i n the plane Z—4.5 f t page 204 a a a a a • ': J » 0 • K a i ; 1 !• a { • T • * a | * J J J , s • • a • m r ~ : 1 • Q p i a i 1 r i b i f _ i > m • • a a a a a a i l 1 l i i i i . T m Z a i * 1 * < 1 • r • I N T E N S I T Y , F T / S • - : > 1 0 0 » - : > 8 0 , < 1 0 0 • - : > 6 0 , < 8 0 o •= : > 4 0 , < 6 0 o - : > 2 0 , < 4 0 • - :>0, < 2 0 •1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E E.4.5 Case 1 Turbulence Intensity, v' i n the Plane Z=.06 f t page 205 o CD -+ O in — o _J to4 _J — CE o 3 CD — -• L_ o LJ 2-*-; i —I I 1 1 1 1 I I 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E I N T E N S I T Y , F T / 5 m «- : > 1 0 0 o - : > 8 0 , < 1 0 0 a •= : > 6 0 , < 8 0 o «= : > 4 0 , < 6 0 • «= : > 2 0 , < 4 0 • - : > 0 , < 2 0 E.4.6 Case 1 Turbulence Intensity, v' i n the Plane Z=1.6 f t page 206 o _J ro-|-_ J ~ CC o 3 c ^ - K 1.0 0.0 1.0 2.0 X , F E E T F R O M 3.0 4.0 5. F U R N A C E T I N T E N S I T Y , F T / S • - : > 1 0 0 o - : > 8 0 , < 1 0 0 B « : > 6 0 , < 8 0 : > 4 0 , < 6 0 : > 2 0 , < 4 0 : > 0 , < 2 0 o • 0 6.0 7.0 8.0 C E N T E R L I N E E.4.7 Case 1 Turbulence Intensity, v' i n the Plane Z-=10. 6 f t t ~ ~ 1 o 1 6 T i 1 < 0 . j i . • < I J 6 • L • «_ • i • • < , 8 • 1 • 1 ? page 207 I N T E N S I T Y , F T / S • - : > 1 0 0 o - : > 8 0 , < 1 0 0 - - : > 6 0 , < 8 0 o - : > 4 0 , < 6 0 o - : > 2 0 , < 4 0 • - :>0, < 2 0 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E E.U.8 Case 1 V e r t i c a l Turbulence Intensity, i n the Plane Y-8.1 f t o to — o CM CO LJ LJ O o o o 03 o CO o LJ •- -O LJ • > CD ° CE m LJ • LJ ™ o d I o CM i rO I I O LO I i i .-5.. * D T T T page 208 I N T E N S I T Y , F T / S a - : > 1 0 0 B - : > 8 0 , < 1 0 0 B «= : > 6 0 , < 8 0 O -= : > 4 0 , < 6 0 D - : > 2 0 , < 4 0 • - :>0, < 2 0 •1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E E.4.9 Case 1 Horizontal Turbulence Intensity, u' i n the Plane Y=8.1 f t . o CM O CD - -CO o or: • LJ OO -z. oLJ t\ ~ O o LJ _ J • CD M O o I D ~ O LJ • > <0H O CO O CE t—< oLJ LJ CM ~ ' L_ o o • 1 T page 209 I N T E N S I T Y , F T / S • - : > 1 0 0 » - : > 8 0 , < 1 0 0 • - : > 6 0 , < 8 0 o - : > 4 0 , < 6 0 ° - : > 2 0 , < 4 0 • - :>0, < 2 0 .0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E E.4.10 Case 2 Turbulence Intensity, v' i n the Plane Y-8.1 f t o CO or LJ f— -z. LJ CJ LJ _J N M O LJ > CD CD CE LJ LJ L_ i IX] * CO o CO o o CO o in o ro o ro o o ro i o i o i r 1 r i < r • i i 1 1 page 210 I N T E N S I T Y , B - : > 1 0 0 : > 8 0 , : > 6 0 , : > 4 0 , : > 2 0 , :>0, o B O • FT/S < 1 0 0 < 8 0 < 6 0 < 4 0 < 2 0 -1.0 0.0 1.0 2.0 X , F E E T F R O M E.4.11 Case 2 Turbulence Intensity, v' i n the 3.0 4.0 5.0 6.0 7.0 8.0 F U R N A C E C E N T E R L I N E Plane Y-14.4 f t o tv. O ID O ID O _J ro - l -_J ~ CE o 3 CM-H-s° O — or ~ L_ o o CE o 00 z: O o Crr LJ CD LJ L_ o o to' o o page 211 I N T E N S I T Y , F T / S a - :>1 DO o - : > 8 0 , < 1 0 0 = - : > 6 0 , < 8 0 : > 4 0 , < 6 0 : > 2 0 , < 4 0 : > 0 , < 2 0 o • 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E E.4.12 Case 2 Turbulence Intensity, v' i n the Plane Z=1.6 f t t 1 1 T 1 4 ' , 4 I ^ L 1 i i page 212 I N T E N S I T Y , F T / S > 1 0 0 > 8 0 , < 1 0 0 > 6 0 , < 8 0 > 4 0 , < 6 0 > 2 0 , < 4 0 >0, < 2 0 o B O • 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N A C E C E N T E R L I N E E.4.13 Case 3 Turbulence Intensity, v' i n the Plane Y-11.5 f t t 1 r 1 * * 1 1 page 213 I N T E N S I T Y , F T / S s o B o D : > 1 0 0 : > 8 0 , : > 6 0 , : > 4 0 , : > 2 0 , :>0, < 1 0 0 < 8 0 < 6 0 < 4 0 < 2 0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M F U R N R C E C E N T E R L I N E E.4.14 Case 3 Turbulence Intensity, v' i n the Plane Y-14.4 f t page 214 1 •1.0 0.0 1.0 2.0 X , F E E T F R O M 3.0 4.0 5.0 6.0 7.0 8.0 F U R N A C E C E N T E R L I N E I N T E N S I T Y , o - :>100 o - : > 8 0 , • - : > 6 0 , ° - : > 4 0 , ° - : > 2 0 , • - :>0, F T / S < 1 0 0 < 8 0 < 6 0 < 4 0 < 2 0 E.4.15 Case 4 Turbulence Intensity, v' i n the Plane Z—4.5 f t page 215 1 I I I N T E N S I T Y , a - : > 1 0 0 : > 8 0 , : > 6 0 , : > 4 0 , : > 2 0 , :>0, D -B -O « D -F T / S < 1 0 0 < 8 0 < 6 0 < 4 0 < 2 0 1.0 0.0 1.0 2.0 X , F E E T F R O M 3.0 4.0 5, F U R N R C E 0 6.0 7.0 8.0 C E N T E R L I N E E.4.16 Case 4 Turbulence Intensity, v' i n the Plane Z=1.6 f t CO or LJ LJ CJ o •«»* • o to • o o o o cn o CO o L J • - -o o in LJ -O _ CO °. CE to f - o LJ LJ oo d i o I to I I o L D . I 9 -r — \ < > < > < > < r t i < > < < < 1 1 • , i < 1 c 5 L « l < t , I ! < L C > < *• 1 ) t » : > « r > > a * * : o o » O 0 g o e • ; i } <) ( > T o • 1 ...11 S <? ° - : . 6 6 7 < R < 1 . 5 • - : R > 1 . 5 • - : R < . 6 6 7 ' 6 > 9 page 216 •1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M B O I L E R C E N T E R L I N E .5.1 Case 1 Isotropy Ratio, R i n the Plane Y-8.1 f t CO or L J r — ZZL LJ o o o o o CO o CO o LJ •-tsi o LJ • > ^ o ^ CD ° CC m (-1 o LJ LJ CM o o I o I n i i o T ~ " 1 0 1 j o < i > 0 0 ( < > 1 < i < ) < • 1 i i L <j i c i < c L — — i • c > e > 8 > e ' t — -e 0 o e t r r-- - - j 9 C. r . o-9 o 1 l • i i 6 o ° - :.667<R< 1.5 • - :R>1.5 • - : R < . 6 6 7 1 — ~ \ 1 r i — ' 6 '—r™ 1 1 1—1 page 217 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 X , F E E T F R O M B O I L E R C E N T E R L I N E E.5.2 Case 1 Isotropy Ratio, R i n the Plane Y-14.4 f t page 218 CO or L J ° o UJ oj _J N O O L J o CD CT o L J rsj i LOCUS OF MAXIMUM SPEED SINGLE ROUND NOZZLE CO o L J L J 0_ cn L J or L J L J O 9.5 8.5 7.5 6.5 5.5 4, HORIZONTAL D I S T A N C E 5 3.5 2.5 1.5 0.5 FROM N O Z Z L E S . F T E.6.1 Case 1 J e t Centerline Speed and Trajectory i n the Plane Y=8.1 f t page 219 • -o -LOCUS OF MAXIMUM SPEED 3INGLE ROUND N O Z Z L E o OO O Q " L J L J D_ o to O o L J 6 ct: O L J 9.5 6.5 7.5 6.5 .5.5 4.5 3.5 2.5 1.5 0.5 HORIZONTAL D I S T A N C E FROM N O Z Z L E S , F T .6.2 Case 1 J e t Centerline Speed and Trajectory i n the Plane Y=14.4 f t page 220 E.6.3 Case 2 J e t Centerline Speed and Trajectory i n the Plane Y=8.1 f t page 221 cn or L J L J O L J CM" O L J > O CD C E t— L J L J L _ o in. • O LOCUS S I N G L E or XPERIMENT ROUND J E T LOT J E T MAX I MUM SPEED ROUND NOZZLE HI IN -r 9.5 8.5 7.5 6.5 5.5 4 HORIZONTAL D I S T A N C E Si? I I 5 3.5 2.5 1.5 0.5 .FROM N O Z Z L E S , F T cn D L J L J o O L d <" 2 L J or L J L J E.6. 4 Case 2 Jet Centerline Speed and Trajectory i n the Plane Y=14.4 f t page 222 CD or L J L J ° o L J o j " ] r-j N o o L J o cn cr o Lj* f a o LOCUS OF MAXIMUM SPEED S I N G L E ROUND NOZZLE o u"> r i L_ O L J L J 0_ cn L J O L J „ L J ° . O o 9.5 8.5 7.5 6.5 5.5 4 HORIZONTAL D I S T A N C E .5 3.5 2.5 1.5 0.5 FROM N O Z Z L E S . . F T E.6.5 Case 3 Jet Centerline Speed and Trajectory i n the Plane Y=11.5 f t page 223 o (D O i n ' CD or L J L J O O L J rvj-_ ! r-j M o o _:• L J O CD cr o L J L J i o in . • - LOCUS OF MAXIMUM SPEED S I N G L E ROUND NOZZLE 9.5 8.5 7.5 6.5 5.5 A HORIZONTRL D I S T A N C E .5 3.5 2.5 1.5 0.5 FROM N O Z Z L E S , F T E.6.6 Case 3 J e t Centerline Speed and Trajectory i n the Plane Y=14.4 f t page 224 Appendix E.7 Variables and Example C a l c u l a t i o n for the Figures i n Appendix E.6 Table E.7 Variables Used i n Models of Predicted Overfire A i r Jet Centerline Speed and Trajectory case 1 2 3 steam load 1000 lb/hr 150 107 107 %0.F.A. 60 31 31 dampers to h o r i z o n t a l nozzles open open closed do f t .471 .471 .443 Ao f t 2 3.50 3.50 1.54 ya f t .164 .164 .144 v f t / s o 357 133 302 v f t / s 8 10.1 10.3 10.3 M xl0~* 3.71 27.8 5.39 In s e c t i o n 7, the models used f o r the p l o t s of predicted o v e r f i r e a i r j e t c e n t e r l i n e speed and t r a j e c t o r y i n Appendix E.6 are presented and described, and the v a r i a b l e s are defined. The v a r i a b l e s used f o r each case considered are l i s t e d i n table E.7. In the following example, sn> and Z are c a l c u l a t e d f o r case 1: X= distance from the furnace plane of symmetry f t X'=8.5-X f t -distance from furnace wall 2 173= o v e r f i r e a i r density  p ' density of gas r i s i n g from the grate s 2 A«=297.5 f t -furnace cross section area Ao«=3.498 f t «= area of o v e r f i r e a i r nozzles (Appendix E.7) page 225 do=.471 ft.= average nozzle diameter (Appendix E.7) 1=10.69 f t . - l e n g t h of s l o t G.G.F.=10857xl03 f t 3 / h r (table 7.1.1) O.F.A.=4496xl03 f t 3 / h r (table 7.1.1) 10857x10s i . ,. . . _ v °297 5x60x60 ' = 1 0 - 1 4 f t / s = v e l o c i t y of gas r i s i n g from the grate 4496X103 V 3.498x60x60 = 3 5 7 f t / s " n o Z z l e v e l o c : L t y w 1 r10.14>2 _ M --.-4 M = 2TT73X^"35r^ 1 = 3 * 7 1 X 1 0 Trajectory of Centerline of Round J e t | ^ = ( 3 . 7 1 x l O " A ) 1 1 2 ( ^ ) 2 - 6 * (equation C.5.1.18), or Z=4.94xl0"4X'2'6* f t . X' f t . Centerline Speed of Round J e t — = 6.3(2.173) 1 /x / v , d ° . . X' > 8do v v ' (X'+.6xdo) o — = 1 X' < 8do (equation C.5.1:3), or v — o S n= ( X ^ 8 3 ) f t / S X ' > 3 - 7 7 f t sm=357 f t / s X'< 3.77 f t Centerline Speed of Slo t J e t 3.498 ro 10.69 327 f t = width of s l o t ___ - 2.48x[ -5- +.6]" 1 / 2x2.173 1 / 2 X' > 8y o J o sm , , _ „ (equation C.5.1:8) or — = 1 X' < 8y v J o o sm= 1305x[ ^ 7 + - 6 ) f t / s X ' > 2 - 6 7 f t sm= 357 f t / s X'< 2.67 f t 

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